100% real. I have a set marked exactly like that purchased off a SO truck in the mid 1980s. Should have "XSM1314 USA PAT 3273430", for example, on the other side, no date code stamp on mine.
The later "A" version dropped the PAT, changed the font (no underscore) and added a date stamp. I can't remember when I purchased the A versions.
The PAT was likely for the Flank Drive - that's about the time it was introduced.
I'd be suprised if anyone would fake an item like this. Yours have the same patina as mine, too. Chrome back then was more of an industrial hard-chrome color, less yellow than later.