Best I can tell, '67 optional shoulder belts were attached at the rear seatbelt anchors. Same as seems to have been done for convertibles. That's what is in the shop manual illustrations. Also there's magaizne photo of a RHD '67 FB in a British magazine that seems to show that arrangement. Would love to see a surviver 67 RHD export model since front shoulder belts were probably standard on them.
Apologies for the OT sidetrack.
I think all '68 will have the upper anchor point, whether or not the nut is welded in place.
Also look for the storage clip which mounts by the sunvisor anchor.
And IIRC, at least in '69 there was a snap button which the loose end adjustment end of the shoulder belt attached to.