do you know anything else about the DX range. I have Yashica DX 50mm 1:2 without serial number
Hello and welcome to the Forum!
Can you please check your lens again for its serial number? Set your lens to the infinity mark and then look underneath to where you can see the orange 2 and white 0.6 marks; between the focusing grip and the aperture ring you should see the serial number stamped onto the barrel, probably starting A904xxxxx or A905xxxxx. It's very easy to miss unless you have the light running across those numbers; as I suspect you're aware, usually the numbers are picked out in white.
DX marked lenses come in three basic variations: the original M42 series of Auto Yashinon-DX and Reflex Yashinon-DX which were a mainstay of Yashica's lenses through the 1960's until the introduction of the DS-M series which sported an advanced multi-coating. In the Contax/Yashica bayonet mount era, there were two and a half DX lenses: the well-known DX 100mm f4 Medical/Dental lens with its internal flash- and modelling- lights; your DX 50mm f2 which is not in any known Yashica catalogue; and the half is a stopgap version of the Reflex 500mm f8 marked DX 500mm f8 (not Yashinon-DX) which had a M42/C/Y conversion mount attached and it's believed that only a few were produced before the 500mm f8 Reflex production run started. It was essentially the Yashinon-DX version so I can only assume Yashica was trying to avoid confusion with its M42 forebear. It's thought that fewer than 50 were manufactured using the hybrid mount; by simply removing a glued screw, you could return the lens to a M42 type. A few years later the amazing ML 500mm f8 appeared which bore no resemblance to any of the previous Yashica mirror lenses.
The DX 50mm in C/Y mount (i.e., the lens that doesn't exist) has stamped serial numbers as biggles3 noted, and since they aren't filled with any kind of contrasting paint that can be difficult to spot. If you locate the large red indexing dot on the barrel, look to the left of it by about 1 1/2 inches on that same band of the barrel, and you should see it.
I've managed to acquire 3 of them over the past few years, and they all have numbers, and none are filled with paint.