Royal Ginger McCarthy wearing 41 in the training pics on the OS. May probably means nothing, but perhaps they've handed the 21 to someone else knowing McCarthy will be off? Can't think why else they's swap him to 41, unless he just picked up the wrong shirt.
Isn't pre-season usually just whatever they have left over? Players wear different numbers all the time. They probably ran out of 21 and are using up the unused 41 kits.