Another wrinkle in the Savage blanket ^^^^^
But one that we've known about for a very long time. There are likely tens of thousands of 99's with steel rotors from 1952/1953. The only reason it's not more commonly known is that most people don't look at the rotor.
Heck, I have 3 in that serial number range in the safe that probably have a steel rotor and I didn't note it down in my notes.