The Carter BBS and BBD were progressions of a single-throat carburetor made by a company named Ball & Ball. When Carter developed those two carburetors, they were named for their heritage:
If you're ever interrogated by the CIA about Carter carburetors, these should help as well:
WCFB = Will Carter Four Barrel (the first four-barrel carburetor) AFB = Aluminum Four Barrel (so named because the WCFB had a cast-iron base) AVS = Air Valve Secondary ThermoQuad = So named because its phenolic bowl keeps fuel up to 15°F cooler compared to other carbs. Originally designed and sold as a racing carb.