If you ever find a singular stopper with a narrow stalk, it can be happily plonked in. :thup:
I believe when these were originally sold, stoppers were optional - they were made in a one size-will-do-all, so a stopper could simply be selected from a batch of them and added to the pot. They are happily interchangable.
Polished round pontil marks were not continued after Michael Harris left or it may have been until after Vicente Boffo left - we still don't know how long he stayed at Mdina before going to Malta Decorative Glass, but it was very probably no more than a few months, if that. They were definitely stopped by then - too much extra work for a small overworked workforce trying to keep up with demand.
Lovely pot, m - loads of stuff going on, and lovely semi-rounded shape!