Swamped barrels were not a catalog item for GRRW. If it was swamped in the GRRW shop, it was a special order or a special project of an employee.Green River barrel. The barrel tapers to the end and then flares out. From what i can see online they stopped stamping the barrels with their name on kit guns in 1977.
It could also have been swamped after-market. The "hat" over "W" is not a mark of any GRRW employee that I know of, so it could be the mark of the person that swamped the barrel.
The caliber and twist stamp is in an unusual location for a GRRW barrel. These were usually stamped on the face of the breech like this.
I suspect these marks were added to the barrel flat by the same person that stamped the "hat" over "W" mark.