You sometimes have to make your own. 12-32 is only about .035” smaller than 1/4. 10-32 SS set screws can be made into vent liners. I seat most to an internal shoulder.
If this is a precarve/pre-inlet stock the pattern stock is bad.
If this was inlet by you then you put the lock to far forward or the barrel too far back. Not possible to sugar coat this.
This is why run of the mill precarves are often a mistake.
If its not a precarve you MUSt draw things out on the stock and then check the parts fit before inletting so everything is in its proper place. Then if things are a little off its only a little. The top surface of the pan needs to be at or just a little low of the center of the flat. And it needs to be placed so that the vent is as far forward of the breech face if a “plain” breech or near the middle of the breech length if the design shown here.
And at or slightly a head of the cavity in the breech.
This is the breech on a 50 cal swivel breech. The cavity in this 5/8 threaded breech is 3/8” done with a ball end mill.
This is like some British breeches from back in the day. It shows the vent seated to a shoulder.
This is a Nock breech internally with Mantons “recessed” breech external.
XXXI is the Nock design. The one I made has shorter threads into the barrel.
As you might see I did not get the vents exactly in the middle of the pan. Its a little harder to do when the breech is made and the lock then fitted to it that if the lock is fitted and the hole then drilled.