Flash Hole Liner, often called a vent, vent liner, or touch-hole liner.
Threaded 1/4-28, stainless steel with lug for easy installation, made in the USA. Drill, tap, and countersink the barrel. Use the wrench lug to install the liner and seat against the barrel. Check the liner protrusion into the bore, remove and trim length if necessary. Cut the wrench lug off using a hacksaw and file the liner flush with the barrel. The vent hole will be exposed when the wrench lug is cut and filed off.
Concave inside, this vent liner brings the main powder charge very close to the incandescent heat of the priming flash. Position the vent on the center of the pan, slightly above the pan to frizzen joint, sometimes called the "sunset" position. Covered by the frizzen when closed, the hole is a window centered on the heat of your priming flash, for instant ignition, without the whoosh-bang delay, seen in many flint guns.
Only the better quality London made guns were factory fitted with vent liners, but nearly all flint guns were later fitted with vent liners, after the vent hole in the barrel became worn from flash erosion, corrosion, or the aggressive use of a vent pick.