All the 'A' means is the machine was made at the Anderson, SC plant. It has no affect on the pricing of the machine nor does it indicate a sense of rarity. Anderson, SC pumped out as many machines as any other Singer plant in the U.S.
While we are talking about funny notions - Singer never made machines to be badged for other companies like Sears or Montgomery Ward. If you bought a Singer, you knew you bought a Singer.
Congratulations on getting your 301! They are fantastic little machines. Your serial number is located on the join of the bed at the hinged section.
If you tell us you got a black 301 for 10 dollars I believe more than one of us will cry.