plate numbers. You can have thousands of blocks with the same number. There are 4 numbers to a sheet of stamps and the same numbers are used over and over again. Most people don't realize that stamps are printed in sheets of 200 or 400 depending on the size of the stamp. Then they are split into panes of 50 or 100. You can have a lower right number or a LL#, Upper right # or a UL#.