I re-read not long ago a biography of Chesty Puller, and he bitched mightily about many of the M1's issued when they headed to Korea in a hurry. NO QC . . . just broke them ot of cosmoline and issued them . . . ungraded. A lot had sewer-pipe barrels, and after the fighting I'll bet a lot more were just as bad. So the rebarreling, even into the 60's, makes sense.
But I still don't get how the '64 got stamped along with SA in Geneseo when it didn't exist yet. Maybe the '64 was already there (when it was rebarreled), and SA/Geneseo was added later?
Just guessing, and realizing it may be unknowable. I no longer suspect it was a counterfeit because counterfeiters probably wouldn't do something so confusing.