From the pictures vonryan's gun, and the little I have heard about these Australian guns, I would wager that standard US barrels, breaches, and recoil mechanisms were never "officially" put on the NF&O guns. There are several reasons for this.
1. From the pictures there is no physical connection between the recoiling parts (breech and barrel) and the recoil mech. Thus, if the gun were fired, the barrel would recoil back and end up in the dirt.
2. The breech ring indicates that this 37mm gun is from a vehicle mount (M5 or M6). And, the breech-operating handle appears to be non-standard. The axle appears to be a light trailer axle. Definitely not a M3 AT Gun axle. Also the surge brake assembly appears to have been replaced by a castor wheel.
3. Another collector in Australia, who had a few of these guns, told us that he rescued his guns from a junkyard. Vonryan’s guns looks like maybe a junkyard operator wanted a cannon to display in his yard. He had a lot of various cannon and military vehicle parts, so he made himself a cannon. This is purely speculation on my part.