I have usually seen them facing towards back of car. This is so after the Belt wraps around it, the open end can retain it as it goes back again.
From what I know, the Shoulder Belts were Federally Mandated for All 1968 Cars. I have also seen a few without, but have heard of instances where the Original buyer did not want them so they were removed and efforts taken to conceal the holes, maybe even a whole new Headliner. Since I never personally did that, I can not 100% confirm those 'Stories'. They might possibly have never been installed. I'm not really sure. But I really don't think it was an exact date running change or anything like that. I do know it was an Option for 1967.
Our last 68 was a 1967 Build Date and had them. I am not sure of exact day built off the top of my head, but VIN was lower than first 10,000, in the early 9,000 range. San Jose Car.
Just my opinion, as always. Andrew