Of course, this is a pretty big digression from the topic at hand so I apologize in advance for the rant...
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Godfrey, Like yourself, the 43mm limited was my favorite lens too! I purchased mine the very first week after it was introduced by Pentax in the film only era and subsequently obtained the two other original "limiteds", the 31mm and 77mm upon their worldwide introduction. They too became "go to" lenses, not because of their construction or unique status at the time, but because of their performance characteristics. Their focal lengths were of course as they appeared on the barrel, since film bodies was all Pentax offered at the time. When Pentax introduced a DSLR, the 31 and 77 retained they characteristics but the 43mm, as good as it was, somehow appeared different to most who compared using this optic on their film bodies vs. the DSLR. It didn't do quite as well at f1.9 and a stop down from max. aperture, in comparison and the incredable 3D look that this lens was capapable of, was mostly gone when used on a DSLR. Thats not to say it isn't a superb lens, just that it changed significantly when its use transitioned from film body to a DSLR.
The 21mm "APS only" lens is another excellent lens and along with the 31, 43, 77, made a absolutely gem of a small compact DSLR system. Sort of reminded me (size wise) or taking along rangefinder body with four gems for lenses. This goes one step further if the 31, 43 and 77 lenses were used with a Pentax MX body....which Pentax itself used to use in advertisments and compare side by side with a Leica M body...small, compact, lightweight and well built, all manual film body. Both the Pentax MX and Leica M were actually shown side by side by Pentax in these Print ads.
I suspected the 43mm RF lens was the same optically as the SLR version (as you mentioned), but sadly for whatever reason, the 43mm RF lens didn't appear to sell well and so Pentax decided not to make any more RF lenses, in any focal length.
I was using Pentax gear exclusively from 2004 to 2008 for my work, and had a relationship with Pentax USA marketing during that period. I had all the Limiteds made up until I sold out of Pentax gear in mid-2008.
(BTW, All SLR lenses in any make are marked by actual, optical focal length, regardless of format. Otherwise the madness of crop factor, EFL, etc wouldn't be what it is. Only Ricoh with the GXR and its camera modules, since they use different sized sensors for different lens options, has a real reason to use an EFL focal length designation ...)
I had heard from many about the supposed poor performance of the FA43 on the digital bodies. I avoided it at first and tried the 31 Limited. I felt the 31 was highly overrated, the FA35/2 was a better performer at a third the price on the digital bodies. The FA77 is superb, if slow to focus (either manual or auto). The later DA70 is just as superb, albeit a little slower but a lot faster focusing. The DA21 was my baby ... a perfect wide-normal for the 16x24 mm format. Finally I tried the FA43 on the bodies and to my delight it was the best of all the Limiteds ... Once I revealed what it could actually do on the DSLRs to the Pentax list by showing my photos with it, everyone else had to have one too.
The 21, 43 and 77 would be a perfect trio. I could live without the 31 Limited entirely ... it's still way way too expensive for its performance IMO.
I suspect the real reason for the failure of the Pentax-L 43mm in the market place was Pentax rather ambiguous and inconsistent position for that market. It was a pretty expensive lens from a manufacturer known primarily, at that point, for value-for-dollar, rather than ultimate quality, and for SLRs, not RF cameras. And of course their marketing has always just plain missed the mark since the LX pro-attempt by being confused, irrelevant and inconsistent.
I no longer care about things from the marketing and promotion of equipment standpoint, my photographic business and personal interests have changed quite a lot in the past couple of years, and Pentax just doesn't cut it for me anymore. Some nice bits they've made in the Limited lenses, a plethora of "almost" bodies, a number of good state of the art zoom lenses encumbered by inconsistent quality control. I'm somewhat astonished they've kept it going so far...
Hopefully the K5 will make a good niche for them, despite the problems I keep hearing about, and they can fix their quality issues. But I'm out of the market on them.