You are right as that was a option in 64. My 64 did not have factory belts but did have rubber plugs in the floor for the FRONT.They were threaded .The rear has dimples where you can drill for the thru bolts. My 65 came with front and rear factory belts.
Well, for all you youngsters (lol) I purchased by 1963 426 Max Wedge Lightweight BRAND NEW in 1963 (I was 21 years old then) and it did NOT have rear seat belts.The car was a 2 Door Hardtop Belvedere Aluminum Front End Factory Race Car. 1 of 11 built as a 2 door hardtop. Don't ask me why I ordered a hardtop rather than a Savoy 2 Dr Sedan...I just thought it looked nicer.Agreed, 67 was the first year for everyone all though Chrysler did have them in many 66 models.It's weird that you don't have any provisions for them. My 63 Chrysler didn't come with them, but when I checked under the carpet, the provisions were there.
I believe they weren't standard till 67.
My 66 Belvedere II (bought as a demo) came with belts for 4 front and rear....Agreed, 67 was the first year for everyone all though Chrysler did have them in many 66 models.