Interesting. The original reason I was curious was because I got a new wheel today (Van Dessel flip flop) and bought a new freewheel (Shimano BMX). When I threaded the freewheel on, the four ridges (not sure what the official term for those are) were exactly flush with the inner axle. Thus when I mounted the wheel the four ridges were brushing against the frame. I added a space to clear that and all is well.

However, it made me consider or think about whether or not freewheels came in different thicknesses.