I see one of our usual suspects is selling what are ostensibly two remote key fobs for “'90-'91 XJS” … I thought there were no key fobs/remote locking or factory security systems for the XJS until MY '93? 
I know Superblack, my '92, came with neither and the Jag factory repair literature seems to not mention them until the '93 MY update/supp (as optional that year).
Wow … well I take it that was an option that MY? Possibly a “dealer only” (i.e. installed) one? How do we know whether we have that option with our car? Or do I need to start looking for a fob for Superblack? I wonder also why the Jag factory lit makes no mention of it, e.g. the lay out, how to troubleshoot the system, etc. … 
What does your 1992 XJS Parts Catalogue say about it?
Paul
I don’t have a copy of the actual factory parts catalog, but assuming the Classic website reproduces same (which I have always assumed), it does seem to indicate that Jag did have a security system before the 188 VIN # series (i.e. face lifts), although it doesn’t show how far back in MY or whether it was optional (note #4, the key fob):
This is what the factory supp. for the '93 MY provides, re: security system:
So how does this security system differ from the pre-'93 system?
Timeline: My XJR-S was registered in the UK December 1992, VIN 186321, it was a special order build, having a non-standard colour. Upholstery dates are around June/July, I guess it would have spent some time with TWR before being delivered to the customer. I’ve attached the manual for the security system. It has the interesting feature where you can give the parking valet just the key to drive it, and be able to keep the boot locked separately. The middle of the three key fobs in my previous post shows the one that allows you to split the key and remote for this reason.
Another thing to trace back, parts wise, would be the other end of the security system. RHS of the boot, a bunch of fuses, a control module, and a 12V lead acid battery backup.XJR-S_Security_System.pdf (1.6 MB)
Having now, for the first time, properly read the whole of that manual I saw the words “6 volt battery” backup, but I’m now looking at a 12V battery attached to the control unit in the rear - huh? I’m sure it is well past its sell by date (I never use the alarm locking), but something’s not quite right here.
Thanks for posting it, Cosmo. I noticed right off the bat I should be able to tell whether my system is equipped for fob/security system usage if it has the tiny light on the end of the dash on the driver’s side that blinks when “on”, as shown in that manual (vs. near the cig lighter in later face-lift MYs). 