Thanks for the replies. It might remain a mystery after all

i found these Winchester model 21 barrels on ebay. Clearly marked "Atlas, Chicago". Most Atlas over unders I have encountered were made by A. Zoli (Antonio?) They may have in fact imported barrels for American guns on order, not sure. I seem to remember Atlas was a machine shop in Chicago, Ithink they stopped gun sales around 1971?. Anyway the choke and rib markings look close to your Parker unknowns

They do. Thanks Gary

Thanks for finding those barrels Gary.
Steve Tasitritsis 'ATLAS ARMS', was at 2704 N Central, Chicago and (later) 7952 Waukegan Rd. Niles, Illinois. The replacement barrels for Smith and Parker guns in the 50s and 60s used tubes made by E.S.C. Vickers (England) and were fit and finished by Armaf in Belgium.
If the Winchester 21 barrels were Belgian or Italian they should carry proofmarks. Very interesting. And what was the source of the "Special Nickel Chromium Steel" tubes??

Years ago I had a Parker with Vickers replacement barrels. I do not recall any Stars on them, but it has been a long time. I went through my saved photos and looked at the gun. There were a few other details about them regarding the extractor and the rib extension that makes me thing that is NOT what Mills has here.