Steve,

The seat belts in 0091 are probably a later addition. No mention of them on the original build list. The seat belt is the 74 to 76? version with a different B style hook than the one used for the 72/73 version. The covers look original and were still available after 1973. Not uncommon to see F models 911 with seat belts mounted in later years.

Rgds, Cees