You guys keep in mind that Darne features were available al cart. You could get obturator discs, or mono block barrels on your Halifax if you asked for them, and paid. You could also specify level of proof applied to your new gun.
I've seen some strange things over the years. If I recall correctly, Dustin's gun was built for a lefty. Not common.
That stock mount on Dustin's gun is common to the slug guns, too. There is a good photo of it in Mr. Bodio's article "21st Century Jungle Gun" in the Double Gun Journal.

Best,
Ted