Want to Buy GEM Micromatic Clog Pruf

But where do you put the shim, over or under the blade?

You put the shim under the spine of the blade (so under the blade).

It is mostly useful for the older versions of the GEM-style razors that do not have raised bumps to prop up the blade to get the intended blade angle.
I apologize for the poor photos, but this will give you a rough idea of how much thicker the spines of the vintage GEM blades were compared to the modern GEM blades. The vintage blade is on the left (modern on the right).





Before this vintage GEM blade, there were wedge blades that were more like a section of a straight razor. From what I read somewhere the GEM Junior received its name from using these 'newer' vintage blades that were not wedge-like. Some of the really old razors that took the wedge style require the use of adapters to use modern GEM blades (I think).

After these vintage blades pictured above, came the double edge GEM blade that you can see in the picture below that came with my MMCP (the blade on the left is a vintage blade like above whereas the blade on the right was the 'new' double edge blade that did not have a spine. Therefore the GEM style razors started having raised bumps on the base plate so they could use these new double edge blades and get the correct blade angle (and could still use the vintage single edge blades). So adding a shim allows you to use a modern GEM blade in a razor without bumps.

Incredible, thanks!