man i just went back and re-read the sachs thread not too long ago, I just got done building an engine using that weird euro chrome bore flat exhaust port cylinder and i'm confused as all get out.
from what i understand, the longer d crank is made up for with a shorter piston, which would make sense, in which case the A/B/C would all use the taller piston and the D would be the only short piston.
but i don't think its straight off that simple, because i've noticed that the pistons come in at least 3 different pin heights and i've noticed the same thing you're saying that different models have different skirt lengths even though the pin height is the same. i also think they screwed around with the compression height on the various restricted bikes, making things even more confusing.
i will say, the current setup i'm running, i have a piston that someone chopped about 3 mm off the skirt and it doesn't run for shit. hopefully you didn't just do that by mistake, haha, i think the c/d cylinders might have different port distance and a different intake port location because of the flat port? This is just a SWAG because i haven't measured port timing and basically just threw this bike together out of junk i had laying around that kinda fit just to see how it would run.
unfortunately i 'got out of sachs' years ago and sold all my sachs shit and meant to sell this balboa but then this cool cylinder dropped into my lap and a 505 and i figured what the hell, might as well build it.
if you want any measurements i know i have at least one D piston in a box that i saved for some unknown reason, and i have this weird euro cylinder which was supposed to be a D stroke but i'm running it on an A stroke engine with this chopped up A (?) piston. I think i'm gonna pull it back apart and see if i cant figure out what new piston to order and then do an actual port map with a timing wheel and try to figure out why it runs like doo doo. unfortunately since everything i own (except for this one piston that has 'stock d piston' written on a piece of masking tap e wrapped around it) is from god-knows-where, i really don't know what i'm comparing things to.
my weird euro cylinder 505/a does about 35 topped out with what feels to be pretty decent stock gearing, but it has crummy low end and zero midrange, as soon as you hit any kind of hill it slows to like 20 mph which is totally unacceptable. its got a 15:15 sha and one of those vintage proma gp clamp on chambers.