Yesterday, I pulled a 3 1/2 engine with a Moss transmission from a 1959 MKI.
The 3 1/2 engine is a much earlier engine than what was used in 1959 MKI, correct?
The engine is not totally locked and will turn a few degrees easily. The transmission also has an overdrive.

 

The engine marked 31/2 litre is a 3.4. It's how they were marked.

 

The 3&1/2 liter engine was Jaguars 3.4 ( as has already been said). So it can date from 1948 through 1959. The color of the head and casting marks will tell you more. However an overdrive transmission wasn’t in use until about 58-59. ( I’ll have to check that to be sure .