As shown by the entry from the OED supplied by TRex, the word
shifter has quite a lot of senses not related to spanners. Similarly Wiktionary has 12 senses, but one of these
is "A spanner with an adjustable jaw size". None of the other dictionaries I checked had the spanner meaning for
shifter. However the Collins dictionary has a listing for
shifting spanner:
Australian and New Zealand
an adjustable spanner
Also called: shifter
I don't see any grounds for making the word common.