I have a NOS front brake cable, which I think is for an RM100. It has the number 40220 on it, and according to Partzilla this is a front brake cable for a 79-81 RM100 (58110-40220). I've never had an RM100 so can only assume it was sold to me for one of the RMs I did have (RM125M, RM250T, RM400N).
What this cable might fit I cannot say, but looking at the parts diagrams the RM100 is it... Maybe an RM80 of the same period?
Anyways, $30 postage included (within Australia) if you are interested.
