This Swivel looks to be purpose built for the lock, the touch hole is through the rear of the breech and not on top of the barrel
The two Aussie guns are vented on top the milled area looks more to be for sighting.
Some observations on the rear mounted flint lock on the swivel:
The short tiller places the hand directly below the pan, when fired burning powder and the flash from the touch hole
would more than likely burn the gunners hand and wrist. I doubt that many would take up the challenge of shooting
it many times. It would be interesting to find reports on it's use was this a prototype? or is there evidence of production
