I have lots of Q Vic, inc 1d black, What is the relevance of the different letters in the corners

by Graham
(Gloucester UK )

Got several books full of british stamps, the hand written books info identify them as 1840, 1841, 54, 55, 58, 65, etc etc. collection ends in 1912. Most appear used. Got more 1924-39.

How do I know they are from the year stated?
I don't know where to start, HELP!

Graham