The first XJ6 made its debut in 1968. The engine options included the 2.8L and 4.2L straight 6 engines. About 80,000 of these were built for the Series I (first generation), which lasted from 1968 until 1973. The V12 model, called the XJ12, was not produced in this series until 1972.
Slightly less than 80,000 XJ6's were made for the Series II. For most markets, this generation ended in 1979, but in Cape Town, South Africa they kept building them until 1981. I guess somebody didn't get the message to shut down the factory.
The Series III got a longer wheelbase with a design by Pininfarina. Well over 100,000 XJ6's were made for this generation, which ended in 1989.
Some different naming systems were used for special models made by Jaguar. Some examples are the XJ40, the XJ81, and the X300. If you want an explanation for all these, call your local Jaguar rep and have them waste their time and money telling you why they made no attempt to make this clear for everybody.
These cars are available for rather cheap prices on the used car market. Some older models are considered collectible, but newer models definitely are not. You can find some pretty good deals on eBay.