When we placed our order for a BC 3560 SS, I asked salesman shortly afterwards about due date and fame model number as I want to switch out std pinbox for a 5th AB pinbox. Was given the fame number, then called Lippert up w/ it. They told me where that fame was going and which model it was, 1621 HD EXT. Called Reese up w/ info so as to get right 5th AB and rep never heard of the "EXT" moniker, but IMO must be extended pin. FWIW, I'd say the frame is a 1621 HD. Ask your dealer for frame number (if they have/can get it), then call Lippert.