ALL Panteras had 'three point' seat belts. The '71-72 cars had separate adjustable length shoulder belts that clipped into the seat belt buckles and did not have its own retractor. Instead, there was a hanger on one of the bulkhead bolts.
As part of the 'L' factory upgrades done in '73-'74, the shoulder strap was permanently attached by sewing to the seat belts and got its own retractor built into the rear bulkhead (both seats). This was a factory change and seems difficult and expensive enough to reproduce that trading cars might actually be a better way to upgrade.
Don't know exactly when the later belts were incorporated into production. Early 'L' models still used the optional-use clip-in shoulder strap; my Sept '72 build 'L' is rigged this way. Predatory police often look for the shoulder strap to tell if a driver is using the seat belts; I've had a few such encounters and some just did not believe the optional clip-on system was legal. I'm retired so it's not too inconvenient to go to Court and instruct them, though.