Early military trucks used a round tail/brake/BO light. The bulbs are actually a self contained units. You could check military suppliers like Kaiser Willys, Midwest Military, Fitzpatcrick, etc. I purchased a complete wiring loom for an M38 from MVPC.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine