They can be used on either camera, The M3 has a slightly different "post" at the take up end, more of a cone shape. This "stabilizes" the take up spool ( the take up shaft of the M3 is shorter than that of the M2). The M2 baseplate has a more pronounced "pip" on it and the take up spool actually runs against the flat surface of the "post". I freely change between them and have not experienced any problem. Even a M4 base plate can be used if you remove the plastic "roller".