Series 3 E-Types fitted with the 4.2 XK engine

Check out the third to last paragraph.

Apparently, the Jaguar Series 3 E-Type, famous for its 5.3 liter V12 engine, was also made available with the six cylinder 4.2 liter XK engine.

According to the Jaguar Daimler Heritage Trust, only 2 or 3 were ever made, and none delivered to customers.

You can read the Twitter thread here.