The leather tab is usually found on the service belt. It's purpose was to better secure the left side ammo pouch. It is on the tab that one normally finds the manufacturer's mark and date of manufacture. Usually, but not always, those buckles without the leather tab were worn with the dress uniform. Those buckles are usually two piece buckles, with the roundel being affixed by four small tabs. Hope this helps.
The tabs were discontinued after 1942. If you see a belt with a tab and the leather is stamped 1943: it's a reproduction.
Cheers!
Rob