My post is a mess but you got the right amount out. Sorry.
I don’t know if it was changed before the XJ40 but the lowers on the XJs can be shimmed to account for wear. The bottom half is removed for that.
When a sealed ball joint is used the bolts are slightly too long as the removable half plus spacers took up a few mm more.
I think the top ball joints are always the same item, some have the nipple added and some don’t, but the part may be the same. Although I’ve had cast ones that looked much older than any XJ as well as ones with flat, machined sides. It’s an old design that works and probably compatible to many more cars.
I do like how they’re all compatible (but beware: the last time the lowers had a slight mismatch and I had to drill out one screw hole to get it in; probably got too cheap!)