Birmingham Daily Post, Sat 1 April 1939
In a column headed "A CHANGE IN RESERVED OCCUPATIONS" appeared (among a host of occupations being removed from the list of reserved occupations) -
"MISCELLANEOUS ENGINEERING AND METAL TRADES WORKERS
Assembler, hand (small components; solderers, soft solder; filer; coil winder (light), bobbin winder; viewer, tester, inspector, examiner, gauger (not highly skilled).
CUTLER, SCISSORS MAKER
Grinder, machine; table knife cutler, hafter; serrater; spring knife cutler, hafter; razor hafter; setter in; scissors borer, hardener; scissors filer, bow dresser; scissors putter-together; ..."
I have seen the the term "filer" used with "Motor Cycle Engine filers" and "Cycle Frame filers".
"Filer" is what it says on the tin - but the trade could vary.