My friend still has a Wayler with this system.
A black cylindrical bung goes into the hole and you turn a threaded collar above it which squeezes the bung and makes it fatter.
(In the same way that a wine bottle stopper works

)
When I used it it was difficult to get it to grip and then difficult to get it to ungrip.
I'd be tempted to permanently attach whatever mast foot you want by filling the hole with Araldite Epoxy Resin and pushing the mast foot thread into it
(after adding some nuts along the mastfoot thread length for the epoxy to grip)
I'd go for the back hole as the centre of effort of more modern sails has moved forward towards the luff.