I just looked at two heaters, a 54 and a slightly earlier one I believe. The 54 has a round "spring steel" clip on the end of the shaft and no set screw. It uses a "d" shapedmotor shaft and the clip just keeps it from sliding off. The heater that I believe to be earlier had a screw that was accessible through the vanes of the squirrel cage. Both heaters are Harrison. The 54 has the side trap door that the earlier one does not.The heater cores are very similar but not matching. The earlier one has a thicker core and 4 screws holding it top and bottom and deeper tanks. The 54 with the door only has 3. I think that they would both mount in the same opening as the brackets bend opposite directions to make the mounting points the same. My 54 core is bad so I think I am going to take both to the radiator shop to see if they can't make the 4 bolt core with the 3 bolt tanks. I hope this info helps you out with your project.
