Holley 5200 and 6500 series carburetors were Holley-produced versions of Weber staged two barrel designs. Ford and Mercury used the 5200C and 6500C; AMC and General Motors applications used the 5210C and 6510C; Chrysler, Dodge and Plymouth used the 5220C and 6520C.
Carburetor numbers are stamped in various places on these models. How to find Holley carburetor list numbers
The 6500 series were feedback (computer controlled) carburetors.