Have you ever found a shim between the flywheel and the crank flange? I have found one of tin and one of paper in different engines. Not what one would expect to get correct magnet gap.
I have also seen one piece shims covering multiple holes and individual shims to be used on each hole. All were metal.
These shims were between the flange on the crank and the flywheel where the flywheel mount bolts are. Not for the field coils.
Yep. You can find all kinds of "fixes". I have a collection of them that I have found over the years. I wouldn't use any of them. IMO.
When Martynn Vowell were helping fix and teach Derrick Pang's Runabout we found a paper shim. It was lined notebook paper.