I did some Googling on this subject.  The PDF document linked below summarizes the seat belt timeline and matches what I found in text articles.  Most of those articles focused on domestic cars and did not mention imports.

Succinctly, belts by law didn't show up until 1968.  That is not to say that some car companies (like Volvo in particular) didn't implement them earlier.  
https://www.michigan.gov/documents/msp/Seat_belt_timeline_03_web_386202_7.pdf

I've had two 67 cars with rear belt provisions 

I think the factory belt mounts didn't come in till MKII? I've not seen them on MKI shells that I've seen?

How many of you are members of the Mini Cooper Register? Of those, how many read Simon Wheatcroft's Mk I article in the August 2016 MCR magazine?

I can't post the photo from the magazine (someone else, maybe?) but one paragraph from his article is most interesting.

"There isn't a great deal of difference between a US car and a British one except that showing just how far ahead of the rest of us the USA was in terms of safety, the US spec cars of this period already had to have mountings for rear lap belts; the fixings are visible in the boot and the rear wheel arches."

The first part of the article has some truth to it as we know. The safety issue ended up banning Minis built after December 31, 1967.

It's the second part that has me scratching my head. Maybe a state or two required rear belts during the Mk I time frame. I know Washington didn't.

Ideas? And take a peek in your car if it's a Mk I and US spec to look for seat belt reinforcement.