Your first problem would be to unscrew the extension without damage, and then fit it to the new barrel, but the bead has to be on top, so lathe work is almost a given.
Then it needs to have the fit into the frame made proper, and the adjusting sleeve that you have will probably not work right, either.
Then you need to make sure that the barrel chamber aligns with the specially mated chamber ring that we hope hasn't been overly cut large or offset to fit the aftermarket barrel, or else you need one of those to be fit, also.
Parts possibly needed: adjusting sleeve, chamber ring
Tools possibly needed: chamber ring fitting set
Tools almost certainly needed: barrel extension wrench, lathe
Experience is necessary to minimize the possibility of ruination of something, and lucky amateurs will have a loose barrel and/or one that doesn't extract, and may feed with difficulty with a mis-aligned chamber.
Yes, I am a Model-12 specialist, and have plenty of spare parts.
Right now I have just posted a Black Diamond Model-12 for parts salvage, but if someone wants to get the complete set, realize that the frame has been battered to the point of being useless without a rebuild. This has very nice figure in the wood and a solid rib 30" full barrel.
I would rather fit all of the parts to a replacement frame from the Nickel Steel era, and use the matching extension that hopefully was with the frame.
http://www.trapshooters.com/noframes/cf ... messages=3
There are the pictures and some details, if you care to see.
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