There were two Mugen M 12's I believe. The original MT-12 and the later MR-12. I believe both had the M on the crankcase. The main difference was the piston and the length of the rod. The MR-12 will have a cutout on the bottom of the piston skirt for clearance with the crank. The MT had a longer rod and so it did not require the cutout on the piston.