Carter Numbers 581S to 2177S
Carter carburetor numbers always have the letter 'S' at the end. Sometimes there is a minor engineering change code - A, B, C etc as well. The kit and other parts are the same regardless of the code at the end.
For example, the Carter AFB carbs 3721S, 3721SA and 3721SB all use the same parts, so we list them all as 3721S.