The blade configurations are different. In other words, one system may not be used with the other. 50 amps 125 volt is a 3 wire system (1 hot wire, 1 neutral, 1 ground). 50 amps 125/250 volt is a 4 wire system (2 hot wires, 1 neutral, 1 ground). Note: each device has 3 blades. The 4 wire devices are grounded on the side.
Never alter a plug to make it mate with another. If two devices do not fit together, there is an important reason.