Green River Rifle Works of Roosevelt, Utah did not offer swamp barrels as a regular catalog item, but that doesn't mean that they didn't experiment with making some or that some of the employees didn't make some for themselves.
Someone posted some pictures of a swamped barrel that had a "GRRW" stamp not long ago. The "GRRW" stamp looked ok, but the caliber and twist stamp was on a flat rather than on the face of the breech end of the barrel. I think someone might have added those stamps later.
One of the successors to GRRW, a company named Green River Barrel Co. in Grand Junction, CO did offer swamp barrels. I have one of their catalogs from 1983 and show a few pages from it below. One page has diagrams for their swamp barrels.
Just to reiterate, Green River Barrel Co. was a successor to GRRW. They bought the barrel making equipment that GRRW had used and tried to continue the tradition. They changed some of the specs such as the twist rate for the larger calibers from 1:60 to 1:66 and probably made other small changes in the manufacture of their barrels.
Track of the Wolf carried their barrels for a few years. This is from Track's Catalog #11. There probably are a number of these barrels out there.
