tiffin
I found a reference to the BB marking as being underneath the serial number, denoting that this pre Series 70 Government Model had a finger or collet type barrel bushing fitted to it at the factory. Later this type of bushing would be installed on Series 70 Government Models, starting with serial number 70G01001. I believe your BB marked pistol is somewhat rare and you may want to have this verified with a factory letter from Colt. Serial numbers for 1969 started at 324500C, while for the following year they began with 332650C and ran to 336169C, before being changed over to the 70G01001 serial number.