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Forearm for sure would not fit, at least without modifications. The Montefeltro uses a small barrel ring and the M1 uses a large one that is capable of extended magazine tubes. My instinct tells me the buttstock would fit if they are both the same era. Newer Monte's have different contour than the HK era Monte's. The forearm could likely be carefully opened up using a Dremel, providing there is enough material in that area to do so and still allow enough to be left for the strength. I know Turkish gunmakers started making Benelli clones, and some came with wood stocks that can be purchased for quite a bit less than factory Benelli. I've seen some that swapped over with little to no work. I have no idea if they make a M1/M2 clone, but it might be worth a quick look. Turkish walnut is a beautiful wood.

Good luck. That's going to be a hard barrel to find, particularly in decent condition. I have a search on Gunbroker.com that notifies me whenever a used Benelli barrel shows up at auction. I also have a similar one on eBay. I probably have not seen 2 in 10 years go across the auction that I can remember. The 121's are not particularly expensive models, it might be time and money well spent to pick up a complete gun. on GB, I see them sell from ~$600 up and saw one in very good to near mint condition sell a while back go for $1,600 with accessories, and maybe the box, I can't remember. It was nice. I don't have any real desire for the 121 other than to complete my Benelli inertia shotgun collection, but I thought a while on buying that near mint one but ultimately passed when something on my list came up during its time. Here's a search if you want to use for GB. It searches for Benelli M121 in shotgun barrels, and excluded M4, Shaw, slug, SBE and new barrels (since these aren't made new, but does include used and new old stock). https://www.gunbroker.com/shotgun-barrels/search?keywords=benelli m121&exclude=m4 shaw slug sbe&condition=3&sort=13&pagesize=96

Our 6 position is the best on the market imo, and most competitively priced.