Series 3 (at least the tray top I have disassembled) had the reel mounted on the bulkhead behind the seats.
There are a variety of different seat belt mounting methods for Series 2/2a/3 - have a look at the Optional parts book for Series 2a/3 for example.
Seat belts have been fitted to Landrovers since Series 1 - the SMA had them on their entire fleet of Series 1s in the fifties, and all the Series 1-2-2a I owned in the sixties had them. Just because they were not mandatory did not mean they were not fitted.
Of course, at that time, the inertia reel seat belt had not been invented, but contrary to comments above, a properly adjusted three point fixed seat belt is actually safer than an inertia reel one, although the difference is small. Consider - how many rally and racing harnesses use inertia reels? If you have an accident, for example, while leaning forward, you are less well restrained than with a fixed belt. The increase in safety of inertia reel belts is solely because they remove the need for adjustment.
Three point belts are a lot better than two point, but two point are (a lot) better than nothing. Of course, full harness is even better than three point.
John

