Engine serial number for my MF 35X

[Farmer Scott]

Anyone have any information as to the letter A in my engine serial number of my MF 35X perkins 3 cylinder diesel built in the UK. When I go on AGCO parts manual online I don't see a range of numbers for various parts for the engine serial number with the letter A. Most times the parts manuals use the tractor serial number and I can use it with no issue as my tractor was built in 1964. But when the parts manual refers to the engine serial number I become confused because nothing shows anything with any letters within the engine serial number.

Please see photo below

Much appreciated