This article is from the Paper Money Collecting FAQ, by Bruce Giese email@example.com with numerous contributions by others.
It's probably a web press note. The BEP has been experimenting with
new web presses beginning with the 1988 series notes. These presses
use a continuous roll of paper, rather than the individual sheets
used for the regular presses. They also use a 96-subject plate,
rather than the 32-subject plate, so the plate position
indicators, at upper left on the face of the note are eliminated.
The plate check numbers at lower right on the face and back, usually
a letter and up to four digit number, have been replaced by a one or
two-digit number. (On the regular notes FW in front of the PC number
indicates that the notes were printed at Fort Worth, Texas, rather
than at Washington, DC.) On the web notes the back PC number has been
moved to the upper right.