I'll have a go at it, but TLDR - no audible differences. I'll use the analog department's time line, but add some twists of my own because as much as I like that site, they're not inocent of mistakes and their gospel isn't cast in stone.
It all started in '72 with the TD-160 Mk-1. It had a 10mm pin shaped bearing shaft, which was an update of the TD-150/TD-125 10mm ball point shaft and two alloy platters at a total weight of more than 7lbs. The speed control and cue knobs had brass assemblies, it used a 90 - 110V synchronous 16 pole AC motor with a clutched pulley, and it had a glued together particle board veneered plinth with two pin hinges for it's characteristic slide-on hinges dust cover. The TD-160's 'tonearm included' version had the superb TP-16 Mk-I tonearm with 16.5g of effective mass, magnetic anti-skate and dynamic balancing. The one thing it did not have is a safety post to protect the motor pulley and shaft from unwanted collisions by the hefty platter. It was expensive to make and the Japanese competition was already breathing down Thoren's neck!
At the same time Thorens were making a cheaper version of the TD-160 called the TD-165 Mk-I. There were a number of features that changed over time but all seemed to be orchestrated by bean counters and aimed at lowering the production cost. With that said, Thorens did not clear it's remarkable engineering from the scene but kept performance up to snuff regardless. Starting with the plinth, it was stapled together, not glued. The classic TP-16 tonearm gave it's place to a cheaper version, the TP-11 Mk-I. It had a weight on string for anti-skate, screw-on back and forth CW, no dynamic balancing and it was overall more flimsy looking. Even the gimbals frame alloy seemed different. The alloy sub-platter was replaced by a black resin lighter version, which in turn allowed for replacing the suspension springs with thinner, cheaper, lighter substitutes, but despite the common belief, the main bearing shaft was kept at it's original diameter of 10mm. This is important because the later 7mm diameter shafts only appeared with the Mk-II of that series of turntables. The brass knobs assembly gave way for plastic and the metallic coating of the knobs themselves went away for raw black plastic look. On a bright note, a safety post was added to protect the motor shaft from the hefty main platter but the clutched pulley was replaced with a cheaper version. Surprisingly and remarkably, the TD-165 wasn't a downgrade on performance. Sure it had a lighter tonearm, but that was the times when higher compliance MM/MI carts were quickly replacing the older stiff and heavy MC carts of the 60's, so the TP-11 was actually a better contemporary matching solution.
By 1975 Thorens came up with the TD-145 Mk-I which was nearly identical to the TD-160, except it included an overly engineered (IMO) electronic auto shut-off / tonearm lift mechanism at end-of-play.
Note the production pattern, by this time, was already established for the TD-16X series. Thorens had the flagship TD-160 model, the cheaper to produce TD-165 variant and an additional automatic (to some extent) variant dubbed the TD-14X. This pattern was kept more or less steady until end of production, bankruptcy (one or the other) or whichever came first.
Enter Mk-II... from here it goes downhill for build quality but not necessarily for performance. In '76 Thorens introduced the updated version of the TP-16 and TP-11 tonearms dubbed 'IsoTrack'. Both had 'wands', first the TP-62 then the TP-63 but the TP-16 kept it's magnetic anti-skate and dynamic balancing while the TP-11 kept the weight on string and general flimsiness it always possessed. It was simply a cheaper version of the same specs tonearm and the effective mass went down from 16.5g and 11g of the Mk-I to 7.5g of both Mk-II. Later versions of the TP-16/TP-11 IsoTrack were designated Mk-III and included an all black look and a TP-62 wand for the TP-11.
So by '76 we have a little twist in the chain of events. First, we have a flagship TD-160 Mk-II, then a new model, the TD-166 Mk-I and an auto version TD-145 Mk-II, all with IsoTrack tonearms, but we also have a TD-165 Mk-II. The later had the shaft diameter downsized to 7mm coming down a resin platter, but that's not all there's to it. All of them had the plinth swapped for a lower profile with new plasticky and breakable hinges that were used to grip a new lower profile dust cover. The plinth was covered in vinyl laminate, gone was the veneer and gone was the classic iconic aluminum dust cover support strip at the front of the plinth. A new low voltage "Sonceboz" motor was introduced on some of the later Mk-II versions which meant the entire power supply wiring hub could be eliminated in favour of a 16V/140mA "wallwart" power supply adapter, which coincidentally solved the multi-voltage issue with different markets. It was also a much cleaner solution for the 220V version which had a shunt resistor installed to 'burn' the excess voltage off as heat. By that time all knob assemblies on all models were plastic and the clutched pulley disappeared from all but the 160 primary models. Around 1979 the TD-166 Mk-II came to birth with the auto variant, the TD-146. Both had the TP-11 IsoTrack tonearm with the TP-63 wand. The 146 came with the 160 inherited silvery control knobs (the 166 didn't).
1982 was nearly the last of the 16X series iterations with the introduction of the TD-160 Super and the TD-147. The super was basically a Mk-II with dampening material stuck to the innards of the chassis, steel dust cover hinges and a heftier thicker plinth that had a substantial bottom cover instead of the earlier masonite plate. It was a very good variant and it also promoted (or rather demoted) some more changes to the 'lesser' models which are way less known. One of which is the TD-160 Mk-III. It wasn't dampened as the Super, it had the regular Mk-II plinth and a Sonceboz motor with the wallwart adapter, but it also had a firstly seen alloy sub-platter with a 7mm shaft.
Later on in the 80's there were Mk-V and Mk-VI of the TD-166 and TD-146 these came with resin sub-platters but they were already too far off the classic TD-16X series and included knobs from later and cheaper series of Thorens turntables and a different tonearm (TP-50) that wasn't a direct descendant of the TP-16/TP-11. These were still available for consumers as late as early 90's.
To sum this up and answer your question, It appears Thorens was influenced by bean counters directives to cut corners and lower production costs since as early as 1972. I had owned (and still do) nearly all the flagship TD-160 variants since the Mk-I and at least two variants of the TD-165. With sincere honesty and to my deepest regrets I cannot testify to any audible difference in performance with any of them. Sure, there's the 'minor' issue of different stock tonearms but that's tossed out the window when I swap back to the classic TP-16 for all variants. I never liked the TP-11 (and it's successors) too much, but the TP-16 IsoTrack is a very good tonearm that I use on my TD-125. My conclusion is that despite the constant economic pressure to lower production costs, Thorens engineering did a pretty good job at keeping the essential design right so performance is affected in the least significant way. Who cares what the assemblies of the control knobs are made of - right? Plastic, brass, kryptonite... That really doesn't make a difference does it?! But surprisingly, even the reduction of the main bearings shaft's diameter to 7mm doesn't seem to make a difference. In fact, my Mk-III TD-160 with the alloy sub-platter and 7mm shaft seems to have the best responding suspension system of them all. You may think a resin platter is a serious downgrade done on the sole intent of saving money, but as it turns out, it may just as well be less susceptible to resonances and allows for lighter suspension springs that improve the suspension reactance. Who knows... I admire the Thorens engineers who managed to get things right regardless.
The difference between a 165 and a 166 Mk-II would depend on the mark of the 165. With the tonearm question out of the way, the general principal is earlier equals better build quality. For north americans with a 120V power grid, I'd recommend the 165 with the original 110V motor. Europeans owners should mind the shunt resistor in the power supply that may need swapping to 10KΩ or just go for the 166 with the power supply adapter. The other general principal is that if you stick to general principals you're bound to miss on a whole lot of fun stuff. They are both the same soundwise.
