A little "Googling" and here's some Fed Reg's.
I added the color for effect.

"Federal Motor Vehicle Safety Standards and Regulations"

Standard No. 208
Passenger Cars (Effective 1-1-68)
Lap or lap and shoulder seat belt assemblies for each designated seating position. Except in convertibles, lap and shoulder seat belt assemblies are required in each front outboard seating position.

Not on subject but something that comes up every now and then

Standard No 202
Head Restraints - Passenger Cars, Multipurpose Passenger Vehicles, Trucks and Buses with a Gross Vehicle Weight Rating of 4,536 kg (10,000 lbs.) or less (Effective 1-1-69)
This standard specifies requirements for head restraints to reduce the frequency and severity of neck injuries in rear-end and other collisions.


The link for the source of this info is as follows:
Fed Motor Vehicle Safety Standards and Reg's

Very interesting document.
Hope this will help to resolve some "Fed Mandates and when" questions.

Frank