Another pandemic-lockdown project.
I bought the car when it was around fifteen years old, and have owned it the subsequent twenty. I've been all over it, and I've always been vaguely aware of a black strap in the trunk (or boot, or luggage compartment, depending on who's speaking), but never thought much about it. I assumed it was for tying down luggage, though it was very short, and guessed there was probably another piece I was missing. Mostly I just throught of it was something that always had to be moved out the way.
I was recently the junk yard and took a look at the trunk, in case there was something in better condition than what I already had. In this case I got the deck carpet and the backing mat that was in good condition (my mat was disintegrating). While I was looking around I noticed the black strap, but this one wasn't just strewn across the floor, it was hanging neatly on a hook on the side panel.
When I got home I looked and yes, my car had that hook too, and a slit in the side panel, and I could indeed hang the strap on it. My strap was all twisted up and the buckle and bolt were covered in a mlky surface corrosion. I unbolted the strap and straightened it out. It looked like a filthy old shoe lace. So I ran it through the wash, ironed it flat, buffed up the buckle and bolt head and added a dab of car wax to hopefully keep it looking nice, and hung it up properly. Looks nice.
There is no mention of it in the owner's manual and I couldn't find photos in my brochures. But I eventually found something on the forums. It's meant to hold a bag with the scissor jack that came with the car. I don't have the jack, and AAA largely obviates the need for one. I might buy a roll-up tool pouch, put a few tools in it, and use the strap to hold that.
By the way, some XJs have a strap on the opposite side; mine does not. That was for a small briefcase-sized tool box. Again according to the forums, this appears to be standard on UK and other markets, but was optional in the USA. Considering the price of a replacement original toolbox, if you can find one, I think I'll leave it be.
There's a thread on JaguarForums with the full discussion and pictures.