A WM2 front works out to be a 1.85 inch wide rim, a WM3 is a 2.15 inch wide rim. These are British designations and were probably intended for Triumphs, BSA's, Norton's, etc. The thing that might be different is the pitch of the holes, ie, spoke holes (and dimples) for drum hub rims have a different pitch than disk hub rims because of the difference in the hub diameters, so the spokes leave the rims with a steeper or shallower angle to match the hub. These are basically the same size rims as the 550's had originally, not sure about the 400, and there is a decent (but not huge) selection of tires that will fit.
