I just obtained a Topcor-S 5cm f2, black version, with short focus throw.
Wow! What a beautiful lens...delightfully lightweight, excellent click stops, buttery-smooth focus and almost perfect glass! Pretty sure (given where it came from) that it has never been serviced since leaving the factory.
I haven't shot with it yet but I'm planning to today...
OK, I'm going to go out on a limb and propose that the f2 and f1.8 versions of this lens are optically identical: on my f2 version, wide open, the aperture blades retain just a tiny bit of intrusion into the element area when viewed from the front. I suspect the 1.8 just has a different aperture ring and allows this last ~1mm of the blades to open up. They do look identical in every other dimension...
Can anyone with a copy of one of these lenses confirm my suspicions?