Ok why..... (might a flintlock have a vent in addition to the touchhole)

[Bob Roller]

Here's a picture of my Joseph Lang rifle from around 1850.   When I shoot the rifle, I get a tiny bit of fowling at this vent, but I do not notice any jet of smoke or flame.

This is almost identical to the breech of the Whitworth semi military match rifle I used for 11 years.Platinum plug with VERY small vent but no noticeable fire of smoke.80 grains of 3fg and a 485 grain bullet will run pressures up beyond and round ball gun.The semi military match rifle was a target grade long range rifle with a long forearm and barrel bands.High end lock and sights as well.
Bob Roller

[Daryl]

After firing a LOT of 'rounds', the side of this platinum plate/vent barely has a wisp/trickle of fouling on it, easily wiped off with a finger & barely leaving any colour on the finger.

[mikeyfirelock]

I’ve been at this muzzleloading thing for about 50 years now and remember when this went around before.    The claim back then was the hole produced faster ignition.
That was quickly proven wrong and it died a natural death. I think this was back in the late 70’s or maybe earlier..
Put it to rest……..OR a drill a hole in your gun and see for yourself.
 Mikeyfirelock ( almost old enough to remember when percussion caps were the newest thing!)

[foresterdj]

A blunder no doubt. If it was me (and I could do something this dumb) after a tantrum I might have drilled and tapped it, put in a screw, and told people it was a custom port for keeping the breach clean.

[TDM]

It was a dead end road idea then and now.