How's progress, Lamar?
Maizenberg is the mandatory book on the subject, I think. Google, download, enjoy, if you haven't yet. I've seen it online in English and Russian.
I've noticed similar behavior BTW, that is the rear curtain getting slow, while it was getting better and better as the shutter speed went up. It's illogical, I know. And lubrication helped a lot with the issue.
BTW, Zeiss Ikon filters (~42mm slip-on) fit on lenses with 40,5mm thread (checked with Jupiter 8), and original cap still works, fits well inside the filter.