VWs started to come with seat belts installed as a standard equipment in 1967, before that seat belts were only offered as accessory at an additional cost payable at the VW dealership, where the seat belts were available as accessory.
Not all seat belts installed 67 onwards were retractable units. There were both lap as well as over shoulder options available over the years.
For points where the seat belts are mounted on a bug, read this post on my thread. A little more detail about seat belts in VWs is available on the previous page of the above post on my thread.
The hardware required to correctly mount seat belts on the bug include following special type of bolt/bolting cap that goes into the tunnel on the pan:
Above hardware unfortunately, not only has become hens teeth here in this part of the world but in west as well. Wolfsburg West used to offer repros but that too is no more available.
I have this single bolt/cap with me, thanks to @shahzebulhaq, which I plan to get replicated. If any of you want a set, let me know as I might be able to get these made rather quickly.