All I know about these machines I got from this website, and most of that from Kay Sellon. She says that they all have some aluminum windings, but in the '73 & '74 model years there were more of the windings done in aluminum. Some of those 73 & 74 machines have had the aluminum windings replaced with copper.
I was talking to a guy in My area who has a bunch of SA 200 machines, He says they each weld a little different.
I have a '66 Idealarc 250 transformer/rectifier machine in My shop that has all aluminum windings. The guy I got it from was going to "scrap it for the copper" if nobody would give Him $75 for it. He didn't know it was aluminum wound.