Well I may have answered my own question, as when looking on this page Classic VW auto parts, VW spares, VW accessories it would seem to indicate that you can fit inertia 3 point type in the rear from December 1971 but possibly in your case
dicky mint, you may only be able to put lap belts in as your car is an earlier model. An email query has been sent to VW Heritage on this subject anyway. So I await their response.
Feel free to correct me if wrong anyone.