The BSN (Body Service Number) is a sequential number Ford started in August 1972 to better track a car and it's production revisions. Note that cars before early August 1972 did not have this ID Plate.
The way that DeTomaso stored cars (body shells) prior to running them down the production line meant that say car 4500 might be finished before car 4400. Ford wanted a (better) way to track ongoing "production" changes (like the location of relays).
The BSN was stamped on a plate affixed between the brake booster and the footbox VIN plate in the front trunk. The BSN started at 50001.
(For USA bound GTS cars the number started with 30001)
This first photo is an example of a Factory installed BSN Plate: