Edwin David Bliss
just spotted Rule 19.22 - apparently the assistant driver can move around and do someone else's job but at half the rating. So, is he the only crewman that can move around?
Apparently so ( in game terms ).
Given his Crew Action tasks and his job title, I believe he is the least required member of the Crew and therefore the best possible "Spare" crew member.
[ Perchance the Tank could operate without him at all, so long as no one gets injured / wounded, albeit the Bow MG would be useless. ]