Gents,
Thanks for all the feedback thus far! Here's some more photos of the stock, etc.
Here's the muzzle end of the rifled insert, with the brass keeper. There are no machined cut-outs. It just nests inside the muzzle.
Here's the breech end, and as you can see there's a mating extractor on the insert.
Sorry... I'm in North Central Indiana, about a half hour from South Bend.