Ian,
Quick way to distinguish the 135/2.5's...
1) The old original K135/2.5 has a 58mm front filter and bayonette mount.
2) The cheap Takumar 135/2.5 has a 52mm front filter, a bayonette mount, and the f stops are painted different colors (plus it has Takumar written right around the outside rim!)
3) The oldest is the SMC Takumar 135/2.5 and it's a SCREWMOUNT lens. There are other Takumar screwmount 135's, but the SMC f2.5 is aledged to be the best.
Good to know that there are two version of the SMC Takumar 135mm f2.5 and only the last one has the same optics as the K lens. It is the one usually found but to be sure, it has a single "4" in the middle of the depth of field markings while the first version has only the double 8, 16 and 22.
Andre
