Shoulder belts and locking seatbacks WERE mandatory for all 1968 USA titled passenger cars. Exceptions to the 1968 shoulder belt law were convertibles which still required locking seat backs.
Lap belts WERE mandatory in all passenger cars by 1966 titled as 1966 models.
Google the DOT regs for these years and 1967. 1967 DOT regulations for passenger cars is the same as 1966 with the exception of the DOT requiring a dual braking system with a warning indicator. (Dual chamber master cylinder, low pressure indicator light)
In addition to shoulder belts, head restraints (headrests) which were previously optional for front outboard seatbacks, became mandatory standard equipment by january 1 1969. Early build 1969 model passenger cars up until january 1st 1969 did not have these unless ordered as an option. More than likely this was because of a ruling by the DOT after 1969 production had started which was typically around july - august of 1968, so they decided on implementation of this on january 1st 1969
