Does anyone know what the thread type is for therear hole in the 21" barrel? (by the breech plug)

Thanks

If you are talking about the flash hole on the flinter, its the same as the T/C. Not sure the thread size.

I believe they are talking about the bolster clean out screw. I would just give Kerry a call at Green Mountain and ask her. If you lost the one you got, they might be able to replace it.

I'm talking about the hole on top of the barrel by the tang. It looks like you could mount a weaver style rail using it and the screw hole(s) from the rear sight. It is a flintlock barrel but it isn't the vent, it's on top and to the rear.