Queen Victoria Stamp Question

I have a stamp I'm trying to identify and I'm hoping you good people can help. I'll try to upload a scan soon, but here is a description: Light blue on white paper Queen Victoria stamp from Great Britain. "TWO PENCE" on the left vertical frame, "HALF PENNY" on the right, "2 1/2d" along the bottom. White blocks in the corners with the letters "A, D, A, D" and you go around clockwise from the top left corner. I found it in an album I obtained in an inherited collection. It's cancelled, and hinge mounted. What do the letters in the corners mean? It's mounted in this album, a 1955 Scott International, under 1873-80. I won't presume that is correct, but I see no other locations that look similar. I'm quite curious what I have here, although I don't feel it's something that I can retire on it's value. Any help figuring this one out is appreciated. Thank you!

It is SG catalog number 142 or 157. Catalog value is nominally £40 but it regularly sells at retail for £1. There are a dozen available on ebay right now with $0.99 starting bids. The corner letters are termed check letters and identify the row and column position of the stamp in the printing plate.

Identification numbers as stated above. There should be a plate number on the left and right sides above the letters of TWOPENCE and HALFPENNY... 17,18,19 and 20 indicates printings of 1880 and the stamp will have an ORB watermark (a ball with a cross above).
Plate numbers 21,22, and 23 are from 1881 and the watermark will be a crown.

Thank you! There is a "22" on either side, yes. Awesome. I was digging around on the internet last night and had no luck at all identifying this stamp. Much appreciation.