Since there is a royal flush starting here, I think you should buy the entire lineup from '68 on, Campagnolo made lots of running changes, especially to this key key chromed part. early on there was no limit screw, it was sculpted from the barrel. Then they brazed on the limit screw boss before either finishing the tool to form it or fully committing to the design.
If I remember the shorter later limit screw boss is a through hole, the earlier that extends longer is closed. Easier to tap and clean a through hole than a closed port. It was the production engineer's doing.