inside many casebacks, stands for "Brevet" or patent.
If you are specifically asking about a vintage Breitling my answer would be: I have seen some questionable watches, never iirc one w/o doubts
If you are referring to that recently listed clamshell piece - it's the second clamshell I see, the first achieved quite a high price, so somebody thinks they are correct; I have found no evidence whatsoever that Breitling used Schmitz Freres clamshell cases - but I might be wrong and miss out on a rare and nice piece.
http://en.wikipedia.org/wiki/Gallet_Clamshell