Hi CS,
The penny black was printed in sheets of 240 stamps, each with its own check letters in the bottom corners.
The letters in the top left hand corner of the sheet are AA and in the bottom right they are TL. These were punched in by hand and so have variances (ie: the location and angle of each letter within the square they're in) which enable us to plate them! So, first row AA to AL, second row BA to BL etc etc.
The links rogdcam has provided for you have images of every check letter for each plate grouped together (so, 12 images: the 11 plates plus the second state of the first plate).
There were a very small number of errors when I was last looking at that site and they might have been corrected by now. The site should have the correct images there somewhere!
Anyway, to help you and others, I found this image ages ago (2016 apparently!!!) but I have long forgotten where it came from:

Most of the time you'll be able to easily work out which plate a particular stamp belongs to.
