Thanks. I'm fairly confident I have it correct now.

This is what I did: 65mmsa.jpg

Ideally, I should have done what you have modified the shutter rather than the lens but I don't think my lathe would hold the shutter accurately enough. It was easier to file away the shoulder. I think with the two surfaces perpendicular to each other being in contact, the spacing has to be correct.

I suppose a roll of film will confirm it!