In 1981 the USPS initiated a new system of numbering the plates.  This came about for a number of reasons.  The multicolor Huck press, designed and used by the USPOD, followed by the Andreotti press in 1971, produced panes of stamps containing a series of plate numbers.  Collectors who tried to obtain complete set for each issue were faced with the daunting task of finding hundreds of large size blocks (strips of 20) to complete their collections.  The USPS realized that they were killing the goose that laid the golden egg.  Besides that, the age of the use of computers was leading the BEP to reorganize the way they kept account of their printing.  

Currency, stamps and other documents requiring serial numbers were now being mixed together.  The numbers became larger and more cumbersome to show on panes of stamps.  This led to the use of a single digit stand-in number; a representative plate number, usually 1, was assigned to each plate used for each color.  Thus, if four colors were used to print a particular stamp, and four plates were prepared, then a single plate number 1111 was printed.  Each ‘1’ was printed in a different color, with the red ‘1’, blue ‘1’, black ‘1’ etc.

The first issue to show the use of the single digit system was the Everett Dirksen issue in 1981, Scott #1874.

A. Single digit numbers on sheet panes

Each pane had a single digit number printed in at least one corner of the pane.  If one plate was used for the printing (i.e. a single color issue), then only one number would show.  For multicolor designs, a series of numbers would show; one in each color for each separate plate.