The thread is old, but I think it worths this picture. I was lucky to collect and compare few f1.2 lens including 3 different kinds of Cosina clones. Optically all Cosina's have no difference. Cosina and Revue -- no exterior difference, just a name one. Vivitar -- slightly different as you can see. Vivitar is quite rare, maybe limited production in 74-75.
I also have Pentax 50/1.2. Pentax lens is slightly sharper, but it is sometimes too 'right'.
I also had Porst 55/1.2 before, and while Porst is slightly softer, there are two notable pros:
- minimum focusing distance is about 40-45cm (Cosina -- 60)
- the construction is more solid, and it feels more reliable.