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The caliper is made for a G (or J too if you wanna go finned) and as +edscoble pointed out, B fits but there's a much bigger parallel gap between the caliper and pads which causes rocking or juddering under heavy breaking.
YMMV, B in a G caliper works ok, which is probably why you've not noticed it.
Also, curious why are you using Elvedes pads? Stop gap (pun intended!) during the Shimano factory fire + COVID + Evergreen shortage?
Can't find my G spare pads but this the compatibility list from the J04C.
B series, is it not a b series in a g series body? (You’d notice if there’s a gap on both end of the caliper allowing it to move back and forth a lots)