1966 shoulder strap cab holes

[yelmer]

Hello , can anybody tell me when the started putting shoulder strap seatbelt mounts in the 1966 chevy trucks . I have seen the earliest april of 66. My dec of 65 truck does not have them . I have also seen some that have the caps and no screw/nut hole it was in a july 66 truck . I think they stopped putting they nut in but already had the holes in the interior sheet metal so they cap the hole . Others that i have seen use plugs in the nut holes . Thanks