PW is their abbreviation for Plain Washer, in other sheets of the TF manual it's called FW...Flat Washer. Flat washer is correct, plain washer is not.
(AN960 Washer, Flat.... Approved Feb 1942)
ANSI/ASME B18.22.1 also covers washers. The ANSI standard defines Type A, B & C Plain Washers, no mention of "Flat Washers"
PW and FW are both correct, just depends on which standard they were using for the washers at that time.
http://www.ansi.org/about_ansi/introduction/history.aspx?menuid=1
