The deluxe seat belt option on '71 and '72s built before 12/1/71 includes a round "BELTS" warning lamp on the driver-side lower dashboard.
The passenger side seat belt light/buzzer didn't enter production until 12/1/71, so the early '72s did not have the Government mandated lamp. Along with a buzzer everyone hated, the lamp was a Jan '72 Federal requirement along with other safety and emissions-related items. Our Government was pretty busy protecting us from ourselves!!
The rectangular lamp for the Mustang is D2ZZ-10B925-B, ID#(D2ZB-10C859-AA), which is replaced by D3ZZ-10B925-A, ID#(D3ZB-10C859-AA). The D2AB-10C859-AB lamp is Ford part# D2AZ-10B925-A, cataloged for full-size Ford, Gran Torino, Maverick, Pinto, and has a chrome front.
The lamp retainer used for the Mustang is D2ZZ-10852-A. As for the E Bay auction for the '72-73 Cougar XR7, Mustang seat belt lamp, and retainer/housing, you can use the belt lamp (same for Cougar & Mustang), but the D2WB-10D841-AB retainer Part# (D2WY-10C876-A) is a Cougar specific part.
At this point, I would contact tpj71mach and take him up on his offer of the light and retainer from his '72 parts dash!