that was the first project I took on to make my SII car family friendly. All XJ sedans from SI on boasted mounting points for three rear belts, two three-point outside, one lap belt central. I’d be fairly sure that the coupé likewise allows for the third belt (mounting points easy to spot underneath the seat bench).
Original SII belts I’ve seen had different covers for the belt reels and different latches from SIII cars. As the mounting position is identical you may resort to SIII belts like I did - just be sure to get the entire set. Drop in fit and perfect function!
SIII ideally allows you to get belts that are “only” 30 years old. Honestly, as a part of the rear outer belt always lies over the top edge of the rear seat squab (and we all know that the leather is typically burnt in this area always exposed to UV radiation and heat) I’d very carefully check the state of the belts you get. With some sense and judgment you should be able to decide whether to use them or better not. Even on Jaguar sedans the rear belts most times have been used very seldom. Check the edges of the belt for signs of wear and - in rolled up state - check whether the part running over the seat is in any form harder or stiffer than the rest of the belt. If so, better let go …
75 XJ6L 4.2 auto (UK spec)