I was under the impression all North American market Range Rovers had electric seats, but I guess only the Hunter edition didn't. A little research revealed that they only made 400 Hunters in 1991, which is probably why replacement seats are so rare:
Britain's Range Rover has come down the mountain for 1991 with a ``no-frills'' but still elegant version of its luxury sport-utility vehicle that has been carving a small niche among affluent Americans since 1987. Range Rover says its ``Hunter'' edition is intended for ``value-minded''...
If it helps, you could remove the seats; refer to page 679 (page 34 under Body) in RAVE
With some access to the bottom side, you might be able to identify what's wrong
On my 83 (original US import!) there are no electric seats, which is kinda nice because they still work like the day it left the factory. ONLY the driver seat can recline and only two settings, the passenger seat is fixed angle no adjustment. Unlike the 2 door model the seatback does not tilt forward but it does have the seatbelts in the back rest. The covers are Velour!
To be honest the seats are very comfortable and I don't really miss not being able to adjust the backrest.