I've built up a collection of shims from the jupiters, and other parts lenses. Mostly left overs from converting Sonnars to Leica mount using Jupiter-8 and Jupiter-3. On lenses that are close, I've put a cut in these and used as a Split Ring. I had the idea of using a left over barrel as a hole punch. Sharpen the edge, even use the screw in retaining ring as a punch.