When you dismantle the film rewind crank on an FE/FE2/FM/FM2 and probably others, one of the parts that you'll have in front of you is a very small stainless washer, probably 3/16" diameter. I've managed to lose mine and now the crank handle flops around without it. Such a small thing, but not so easy to find a replacement for.
If you have a spare one laying around that you'd be willing to part with, drop me a PM, please.
Not trying to be a smart ass, but any chance you could order something very close?
If you click on the part number a CAD drawing pops up with exact measurments. Not sure they come thin enough but 100 will be less than the shipping.