Would have been an outfield player, wouldn't it?
Number 12 is a traditional back-up keeper number. That said, on taking a second look I've just realised that it was only in 1993 that they introduced persistent squad numbers, so the answer is that number 12 would've been worn by a substitute - I think at the time you could only name one anyway. As for who that sub would be, I guess it depends on the match, thus making it quite hard to determine whose shirt it was.