I replaced my 1950s SM420 with a 1967 model. The new one simply bolted right in so the input shaft diameter, number of splines and pilot bushing diameter were all the same. However, the input shaft is different internally. The old one used a spanner nut as a bearing retainer and the new one uses a snap ring. Also, the new one was machined for a proper oil seal. So, the part numbers would be different.
