Plate block

The "control" consisted of a letter in the selvedge of the sheet and was introduced as an accounting method by the printers due to the very large number of low value definitive stamps they were printing.

Then plate block collecting changed in the US due to the addition of up to eight multi-digit numbers which represented different colors used to print the stamps.

[4] This lasted for about ten years before the post office reverted to the traditional single number for most stamps.

[4] In a press release dated Dec. 10, 1980, the postal service announced a new plate numbering system that would, except in cases where more than four designs appear on a pane, "establish a plate block as consisting of four stamps regardless of the number of inks used or the press used to print the stamps.

They may also collect all of the block positions, such as the numbers of each corner that exist after a large sheet is quartered.

A cylinder block of the Sudan stamp jubilee issue of 1948
A plate strip of three of the 2-cent US stamp of 1895 includes the BEP's name.
A US plate block from 1972. Note the addition of a number for each color. This increased the cost of collecting plate blocks. That, plus the format change, discouraged many collectors.