Prompted by @sbobev I’ve dismantled my steering wheel set up to document part numbers, tear down, wiring etc. Some history: my car is a 1992 XJR-S, but when purchased it had a standard XJS airbag steering wheel, not the correct 4 spoke jaguarSport item. That would be Jaguar part SPC1124 (+trim code) - there is a two spoke alternative listed as SPC1011 (US market?). A long time trawling through eBay eventually got me one in black, the Momo part number seems to be stamped 16400, although it doesn’t come up in any internet searches.
Next we need a boss to fix wheel to column. Two choices - up to VIN 185819 you want SPC1122, after that SPC1307. No prizes for guessing that both options are made from unobtainium. More time on eBay eventually got me a complete jaguarSport steering wheel + boss from an older XJR saloon. I’m not certain what the specific Jaguar part numbers are, but the Momo wheel has markings TYP M 38, KBA70056 and (I guess a date code) 11-89. The boss is stamped K2206. The Jaguar parts catalog has the XJR wheel down as SPC1014 and two choices of boss - SPC1047 and SPC1123.
Now we just bolt it all altogether - sadly no. The XJR boss is not quite right. The spline fitting is correct for the column, and the 6 hex bolts match the wheel. But the horn ring + self cancelling mechanism is different. I think the XJR has the horn press in the middle of the wheel, while the XJR-S has horn buttons hidden in the top arms of the wheel. I had to take the horn ring from the original airbag wheel, and re-drill it so that it would fit on the XJR boss. It’s not pretty, and the guy who bought my old wheel, with the wrong horn ring, was probably shaking his head when it arrived on his doorstep, although he was likely only interested in the airbag.
PS. I ended up not using the proper XJR-S steering wheel I spent so long searching for. I prefer the “free” steering wheel I got from the XJR with 03:15 hand placement, rather than the 02:10 that is forced on you with the correct item. It is also a little smaller in diameter and a bit thicker. One stroke of luck, the XJR wheel had the horn buttons, just not wired up, so I was able to connect it up, just as the original should have been.
It’s all apart at the moment. I’ll delay putting it back together (nowhere to go right now ) if anyone has further questions or wants pictures.