1991 XJ6 - 30K miles - Restoration

A Sovereign would have come with self-leveling rear shocks. I believe all 1990-1992 US spec cars came with self-leveling except a base model, which seems to have been 1990 US model year only. That model had no sunroof and less stainless steel trim such as the window frames on the doors.
Due to the short life of the self-leveling shocks, Jaguar provided a Conversion Kit, JLM11698, which contained all the pieces that were different on the shocks and a bleed nipple to blank off the feed line from the valve block to the back of the car. We used to do the rest of the conversion and remove the hydraulic pump, install the blanking plate, and change the PS reservoir and feed hoses. JLM11698 was discontinued 12/2013 but I have provided all those pieces to a couple shops in the last few years when they had a customer with the rare car that still has the self leveling system.
So if the car drives well, it is likely it received one of those kits. You can read the manufacturers number on the bottom of the shock. It is a Bilstein 24-018050. Bear in mind, the shocks were just part of the kit that replaced virtually everything in that assembly. Springs, the big hat at the top, and several other items are different.