The bren gun carrier used a double pin base on the rubber lamp on the back sign IIRC, as well as the convoy light. The UC part number of C01UC13987 translates into NSN 6240-21-854-7650 which is trade number bulb #82. The description for it is
OUBLE CONTACT BAYONET BASE,B9-G,G-6 BULB,1.438 IN. LG,TRANSPARENT,WHITE,6.500 V,1.020 AMP,500.0 HOURS LIFE C01UC13987
These are two pin bulbs that only had one filemant. One wire was ground and one was power. The base is spec'd as a "bay15d". The giveaway is that for dual filament bulbs, the locking pins will be on defferent levels for indexing purposes, so as to ensure that the brighter filament will serve it's purpose. (ie turn signal is brighter than the running lamp). On a single filament bulb, the two locking pins are on the same plane since plarity does not matter.
Could it be possible that you have found a stash of carrier lamps? Photo below is a bin of the rubber lamps and leads at my work.