Without having the info that you have access to, I would say that they already had serial numbers 840, 842 and so on up to 854 and so for reasons best known to themselves, added the 3 before the serial number to make, say 848 into 3848. Just my thought.

I have noiticed that on several of the Mk1A's that I have repaired over the years, the 'A' has been added as an afterthought

But an interesting subject. It does highlight the trouble with writing a book that I'm sure Clive agrees with! The first person to read it immediately knows more than you do. That's because he knows ALL that you have just written PLUS the tiny,weeny bit he knew before!