To the best of my knowledge from photos I have saved, the 515 with the Red White Blue label came first - the magnet rear stamped Altec Lansing Hollywood, and a grey wrinkle finish on the standard 15" Altec basket. These have lower serial numbers, up to about 3500.
Then came the same basket and backstamp, just with a 515 label that was gold. Serials from about 5000 to 9000.

I've never seen a label that says 515A.

The B then came out as others have said c1955 with a smooth finish to the basket.