Congrats on the find, for some reason I feel like I've seen those IMFs before with that modification. I'm almost positive I have.. but, can't recall where. Either way, that's good they sound great.
Within the last 3-4 years, There were a couple of pairs of IMF TLS80 MkII done with single tweeter and occasionally double tweeter replacements being sold on the east coast (Maryland or Pennsylvania IIRC) that were listed on eBay or US AudioMart (and by Extension Canuck AudioMart) that had similar mods. The listings stayed up for a few months or so. May also have been done to a lone RSPM or TLS50 as well.
Most of these mods, IMO are because of the relative difficulty or sourcing the original Celestion HF1300 / HF2000 tweeters or similar Coles tweeters, and mostly because of the expense of obtaining from the UK. The original drivers Are a complicated dual tweeter solution that was implemented at the time, but as pointed out, there were few tweeters of the day that could reliably cover a wide HF spectrum with good power handling and I suspect there was a reluctance to just use KEF tweeters like everyone else or they couldn't get them at the costs they wanted.
John Wright, the IMF designer, was no slouch at designing TL speakers and crossovers, but he was limited to what was available in the UK market, and like many UK companies, was reluctant to use Euro drivers like the Peerless or Audax rectangular faced tweeters of the TLS50 and ALS / Compact series. I suspect that to sell the bigger home speakers to the domestic market, the use of well known all UK sourced drivers was needed. Later his contract as TDL with ELAC made purpose built copies of various midtweeters and tweeters available for use. Certainly more capable drivers were used in later TDL speakers.
The IMFs also represent Wrights ideas about crossover theory - 12 and 18db slopes with EQ and level matching built in. The drivers do overlap at crossover points but not very far so as to protect the upper drivers against going down too low. The TLS domestic series were somewhat simplified versions of the very complicated RSPM crossovers. And yes, most UK speaker makers of that era used ferrite or steel cored inductors and were well aware of the issues with core saturation, hysteresis, and distortion issues. Most of the designers coming from a BBC engineering background felt that the ferrite inductors, used within their power handling specs worked and sounded just as good as air cores, while the air cores represented a huge investment in cost, real estate, and weight by having to use much larger wire and coils for the same size inductor. And yes, cost always plays a role in the commercial and practical development and production of a speaker. It is the same reason why that era of British designers used NP electrolytic caps in most parts of their crossovers, unable to justify the expense of banks and banks of multiple film caps that would otherwise have been needed.
The other issue for IMFs is the designs represent the trade off of efficiency and sensitivity for response flatness. TL's with those era drivers are inherently insensitive, using relatively "slow" and heavy polystyrene, damped Bextrene and limited output tweeters in a quest for flat power and frequency response. Complicated crossovers also represented insertion loss. British domestic and studio environments were relatively modest and high output was not a huge requirement for most - also, quality high power amps were few in that era and slow to come into the UK market.
Quality UK speakers of that era, with few exceptions, were designed with a typical British reserve and politeness, decrying in review after review any speaker with a pronounced, forward, or shouty midrange like most "HiFi" speakers on the domestic or rock n roll market. Many subscribed to the "BBC dip" type midrange sound and most used Bextrene mids. The more brash late 70's speakers like the Celestion Ditton 66 was designed a lot more like their American counterparts, and in some cases, received the same polite but dismissive reviews that American speakers got - efficient, loud, but not for the "discriminating or sophisticated" British listener. Yes, there was a lot of reviewer bias, snobbery, and UK homerism in those days as any perusal of the many UK HiFi press would show. It wasn't until big power became available and speakers like the Meridian M2 and M20, KEF 105 and 104/2, and various Linn and Spendor models would show stout output with smooth frequency response.
So I had to order individually. Just finished up the second speaker before I came to work and