First of all 1957 was when the power shift  wheels were available 56 on the power steering. What you must also remember you could order both of these and install them after the tractor was built. Do you have a serial number?
A firm number well there is no such thing. The power steering pump on the first ones is a little different also we had a few 56 diesels around that have a different starter and of course they do not have key start either.