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You should contact Charles Talbert at M-Series re-build, even though M-37s are their thing, at the very LEAST he could most definitely point you in to the right place.
Go to vintagepowerwagon.com, I bought brake parts from them, they have the stuff you are looking for.
OR look up M-series re-build, Charles the owner is very knowledgable and will be able to steer you in the right direction.