First, the '56 starter would have been 12 volt and the '51 starter would have been 6 volt. If the '51 starter were used on the '56 engine the '51 flywheel would have to have been used. That's a posibility.
You don't say if your system is 6 or 12 volts but assuming it is 12 volt and you do have a 6 volt starter and you continuously cranked it with 12 volts, it is probably bad.
