I had a chrome fork built of the same Tange tubing (or blades, in this case) that was labeled "MTB", but that fork was by no means a Mtn. bike fork, it was for 700C road wheels. I think Tange was convinced that the tubing would only be used for Mtn. bikes when they made the decals, since that's what the people were buying.
BTW, I might have a dustcap for you, PM me and I'll start hunting for it.