The Mark X came with a 3781 cubic centimeter engine or a 4235 cc engine. I have no idea why a car aimed at Americans would have its engine measured using centimeters. Once again, know your market.
Transmission options included the manual, manual with overdrive, and automatic. These were built during the years when Jaguar still embraced the manual transmission.
The Mark X was renamed the 420G in 1966. I have no idea why they would take a vague name and make it even more vague with a mix of numbers and a letter, but Jaguar was obviously pinching pennies in the marketing department back then. I know, I know, the 420 means 4.2L engine. That's genius, or something like that.
These cars and related models are popular as funeral vehicles in some places. You can find them on the used car market, although they are kind of rare because they haven't been made in over 35 years. You'll find some listed on eBay.