I have this problem with a Spotmatic, at 1/1000th of a sec, it was about 50/50, the frame would lose about a third. I learnt this the hard way too as I had reskinned the leather when it came in from the mail, the CLA was going to be more than the camera so I'm trying my luck with another Spotmatic, I've yet to finish the first roll so my luck remains to be determined.
I just picked up a Pentax S1a the other day and wow, you can see looking through the camera at a light from the back with the film door open. You should be able to see an entire frame regardless of speed if the timing is unaffected. On that S1a, shutter speeds At the unmarked 1/1000th speed, 3 of 4 frames were totally dark, the remainder showed barely a sliver. At 1/500th, most of the test shots were half frames with the occassional 2/3rds of a frame showing through. Even the 1/250th speed had the occasional partial blockage.
I'm wondering if this is a horizontal shutter issue more so than a vertical one. :/
(
guess I got some fancy book ends now)