As far as I know, the first barrels Ruger described as hammer forged were the ones used with their M77 Mark II Magnum and Mark II Express rifles, since those had the quarter rib forged onto the barrel as one unit. I don't know for sure, but with the investment in that process, they probably expanded to the other lines for their barrel needs. The M77 Magnum and Express rifles came out in 1991.
Ruger's hammer forged 10/22 with spiral finish bull barrel came out in 1995.
Chet15