When you say Barrel Extension (Model 12, 42, 97) ..this is what I immedietely think of and this is what Winchester called the Barrel Extension.

[Linked Image from i.ibb.co]

Is this what you want to install on a stripped VR bbl so it can be then used on a M12 TD frame shotgun?

...Or by Bbl extension are you saying a cut off portion of a M12 bbl that includes the muzzle to use to put the VR bbl back to orig length. Thread and screw onto the muzzle in place of the former PChoke.

If it's the former,,the Winchester Bbl extension.
Since the stripped bbl needs to screw into the Bbl extension, The added lug for the VR sitting on top of the bbl is right up against the front edge of the bbl extension.
The VR itself is easily removed by punching the pin(s) out and sliding the rib forward and off of the posts.
But that still leaves that rear attachment lug in place of course and in position. The lower magazine tube portion of the bbl extension swings around and hits the VR lug and stops everything right there.

On some installs, the lug spans the bbl & bbl extension joint when installed so you can never take the bbl itself out of the bbl extension w/o first unsoldering that back lug.

Those added lugs are Hard Soldered in place ('Silver Soldered' to us older folks).