Comment by xiphophilos:
The history of the "Americana" series and the reason and date for the A prefix has now been added to
the series profile.
LaurenceD's copy without A prefix on the A side, but the original A-prefix on the B side is from the transitional time in late October 1947, when Capitol had just dropped its letter prefixes again (starting in July 1947, they had used A for "Americana" and B for Pop) and were using up already printed older labels.