Th tricky thing is C's are pretty uncommon. Often they weren't even labeled as a C specifically, they just don't have any designation. just 505/1_
So what happened is there's really shitty information about em based on "best I can remember" sorts of speculation.
Anyway I was looking at a bunch of factory parts manuals before and we found there were 2 piston sizes (technically 4 but it was standard and an overbore option for each)
There was one piston that was like 50mm tall listed for C and D, and one 3mm taller listed for A and B. they're all same bore. I also know that putting an A piston in a D jug hits the head, and putting a D piston in an A jug looks too low.
to the best I know, there are no topend differences between 504 and 505, but the 504/1c is even more rare and unknown so maybe that's a unique one.
I've also only got that one year's factory manual and maybe a dozen sachs engines worth of experience and that's hardly exhaustive - likely different years could've seen differences too. Especially later sachs and the clones made by SIS and the clones used on Kamasura also add confusion
and finally, Athena I'm pretty sure sold their piston as a stock replacement. it's the one that comes with the Athena 50cc cylinder : http://www.treatland.tv/sachs-athena-50cc-piston-p/sachs-athena-50cc-piston.htm but I've heard (only anecdotally) that this is a good replacement for sachs, with no definition A vs D or anything.
So yea, this doesn't help much but hopefully frames some more of the existing knowledge.