I did find the code 75. According to the "Catalog of Firebird ID Numbers" a 75(4) in 68 is for "Belt, Front Seat Shoulder."
Thanks for the info, because my car did come with them!
1968 400 Coupe, verdoro green, black vinyl top 1968 400 Convertible, verdoro green, black top 1971 Trans Am, cameo white, auto 1970 Buick Skylark Custom Convertible 350-4(driver)