Comment by xiphophilos:
Capitol added a B-prefix to its pop releases between July and October 1947, and a A-prefix to its Capitol Americana series.
The earliest B-prefixes I have seen so far are
B437 and
B438 (both July 1947).
Here are a bunch from August and September 1947:
B450,
B451,
B452,
B453,
B454 and
B455.
Starting with
B-461, Capitol added a hyphen between prefix and catalog number.
The last B-prefix release may be
B-466 (late September 1947).
Capitol seems to have dropped the prefix idea pretty quickly and repressed these releases without letter prefixes.