As it stands, it appears the older version has the applied model number "plaque" and black weight on the tonearm, smooth pitch knob. Newer version has screen print directly on the plinth and the silver weight on the tonearm. Unless someone comes up with a hybrid - and at this point, that could've been cobbled together from parts units anyway, so tells us nothing definitive - I'd say they made these three changes - with perhaps more yet to be uncovered - at a specific cut over point in production. Quite likely when the line change from 32 to 31 took place.
Regarding the model plate/screen print, I wonder if at start of production there were concerns that the screen print wouldn't reliably "stick" to the high gloss finish on the plinth. Wouldn't be the first time Sansui had problems with poor screen printed legends on a panel, and maybe they sought to avoid that by using the stick on badge. I had a 5000 receiver front panel I had to replace because much of the legend markings came off just with the light catch of a fingernail, and that was on anodized striated aluminum. The one I installed clearly used a different process as the screen print is rock solid, and even looks ever so slightly different.
The weight I could see simply being a running change for aesthetic improvement. The whole tonearm is silver why not the weight as well? The pitch control knob may have been a customer feedback prompted change, as the smooth knob may have not been as easy to grip. That sounds like an unlikely issue, but I can't think of any other reason (than just for the hell of it anyway) to slip in a minor change like that.
Now, if there was overlap of production between the two lines, it may just be that line 31 (which I'm assuming is where the later production/revised version took place) started with the new parts and line 32 finished out their build with the older parts. Are we assuming that both lines were in the same plant, or not? For that matter, do we know with certainty that these - some or all - were built in house by Sansui, or that a contract manufacturer may have been responsible for some or all of the units built? Lots of questions and no clear answers unless someone directly in the know - say an ex Sansui employee - is reading and posting here. And how likely is that?
Sansui made a lot of running changes, most never documented in any substantive way, and it hasn't been until forums like this popped up and we the obsessed fans of their gear started scrutinizing these old components in great detail that many of these changes became evident to begin with.
At 40+ years on, probably the best we'll ever do is to establish a fairly narrow date range in which the changes took place, whether it was concurrent with the production line change, or if the changes started on line 32 near the end of their run, and were integrated from the get go on line 31.
My next guess would have been that maybe one version was for a specific geographic market but I don't think that's the case because I believe both can be found in the States. Is it possible one was the PX version like the AU-707 (AKA AU-717)? 
...