I wonder whether anyone here can supply the answer to an enigma...cos no one on the Yashica forums has been able to yet. I have just bought another DX 50 f2 lens - so what you rightly say? But this is in a C/Y mount as is my previous one; DX lenses were M42 screw mounts. The only DX lens produced in the C/Y bayonet mount was the 100 f4 Medical/Dental lens with the in-built ring flash (fabulous lens). The serial numbers of both DX 50s postdate my Yashica ML 50 f2s. The coatings are more like the Zeiss T* in colour, unlike the MLs, and the lenses weigh more. So, the DSB single-coating lenses had been long retired, the ML multi-coated lenses were in full production and yet Yashica took the trouble to produce a very, very limited run of entry level lenses that appear to be a hybrid. Why on earth go to such expense when production was in full tilt? And there's nothing I can find in any Yashica literature that refers to the DX 50 f2 lenses in the later C/Y mount. It just doesn't make sense! Though I must say that my DX's are superb performers.