Another DX 50mm f2 in C/Y mount....

Yeah, I can see that scenario of a gap in ML production being the most plausible.  Or maybe the brass at the main office were afraid folks weren't recognizing the ML lenses as better than the DX ones, and made a temporary name switch.  It also could have been a marketing move thing, only sold in Japan.  If sold in some other country, it could happen if there was a hold-up in getting proper import/export documents arranged, or getting the ML designation trademarked.

PF

  Why couldn't I think of that!!

Maybe they had some leftover DS lens bodies, and some leftover DX bezels, or they misengraved some DSB bezels, ... Nah. Quantum entanglement.

Well, i'd guess nobody can give you that answer, Graham..after roughly 40 years...besides that, i am happy with my ML 50/2 & ML 50/1.7 iterations...life is too short to check out too many lenses...or disassembling them.... ;-)

Sadly, the only date-specific historical details I have - and even then, not terribly extensive - centers around the cameras and bodies. Anything lens-wise is hit and miss. However, I can draw on some bottom-up family experience in assembly line manufacturing over 3 generations (i.e, 'shop rats' at GM) for what I think might be a logical possibility.

Mass producing highly engineered products like cars or cameras/lenses, would always need some ramping up over time and in stages to produce them. To skip a step has cascading consequences for everything from fit/finish, reliability and warranties, supply chain corrections, and goes all the way to after-sales repairs and parts. Designs have mockups, then a 'proof-of-concept' working model first. Then after some hand-assembled prototypes, there comes a time where you have to run a sample batch in limited quantities through a real assembly process. It might take years to go through it, and when you're at the last stages in gearing up for a production test run... you'll already have a stock of all the components and subassemblies needed. Whether those would all or partially come from inside or outside - they'd require a fair amount of lead time and expenditure before that point. This might be the part of the process where Yashica's commitment is made to pursue a new line, and the company is still stable to most eyes when the decision to go ahead is made.

If the company is producing cars, there might be 5, 10, 25, 50 of a new model done on an off-line assembly line to test the whole manufacturing process, verify quality control, and do all the process analysis reports that every maker expects to see before 'somebody' signs off on scheduling a regular production run. If it's smaller items than cars, where there's hands-on assembly for everything, there might be a much larger test run in volume. It might be 1000, or 10000+, where you'd ultimately plan on taking what's made and mix them with the regular runs when that stage is finally started. GM did front brake spindles and rear axles like this.

If these lenses were well into the process and at or near the trial assembly stage when things got dicey for the company, the obvious thing might be to recoup what they can from the investment that's already been made. With a bailout on the horizon and a shakeup in management looming, the bean counters tend to take over to some extent, and the end result of the recovery mode might be that X number of test run lenses are released to distributors as the 'next generation' model that's been okayed for early release. No mention is made of pending troubles by Yashica, and there's no incentive to say a word if everything works out. After the bailout is actually done, you explain that restructuring has dictated a new direction in product development, and that the line is 'merged' into something else.

That means there's no shutdown of the existing assembly lines for what GM would call "changeover", where old tooling is pulled out, and the new goes in over a 2 week period of mandatory worker furloughs. There's no further orders for assembly parts on the 'new line', and all of the unique parts made for them are assembled and soon gone out as a one-batch sale. No one at the company wants to offer much for explanations, and there's little need to advertise a line that will never see a real production run.

This is all 100% speculation (and incredibly long-winded), but it comes from being laid off permanently in the early 70's when GM shut down parts of the Pontiac plant I'd worked in. Thousands of workers were never recalled, the Safari Wagon and the Grand Prix lines were later closed. For the employees that remained after the layoffs, they were re-assigned as needed to complete assembly of the parts they had on hand for both model lines. Eventually the remaining plant was retooled to produce the Fiero.