I mean I understand how the shutter works. I've got a few other cameras that use a similar vertical curtain (Yashica TL Electro X, Voigtlander R2A, my DSLRs, Nikon N80)

Just none of them do what you're describing. They all open up a full white frame at 1/1000th.

Except my Pentax Spotmatic which also does not work at 1/500th and 1/1000th. It has a horizontal cloth shutter though, but the effect is the same.

I'm curious to see what the results will be. If the camera works, I will be happy as a pig in mud