There are a whole bunch of differences between the Singer 301 and the Singer 301A, but for simply getting one’s first one from an auction, that bunch of differences probably won’t matter much.
The 301 was in production at the Anderson SC factory when Singer decided to use the letter “A” along with the model number for the machines made at that plant. There were 301s made at Anderson too, from about 1951 to 1953. There were even some 301-1, 301-2, & 301-3 machines made there.
The best place that I’ve found for information on the Singer slant needle machines (301, 301A, 401A, 403A, etc) is at:
http://singer301.com/index.html
About anything that you’d ever want to know about a Singer 301 is there.
CD in Oklahoma