Ruger rings are proprietary, but work very well... they are solid and when locked down, they don't move. The receiver on M77's is stepped, requiring two different ring Heights with the taller ring in the rear position. The rings are designated #3 (low), #4 (Medium), #5 (High), #6 (Extra High). So with the M77's you will need a 3/4, or 4/5, or a 5/6. Ruger models 77/22, 77/17, 77/357, 77/444 and No.1's have level ring mounting, requiring the same height ring front and back. Rings with a "B" designation are blued and rings with a "K" designation are silver... if they are Matte finish there will be an "M" in the designation also.
My only beef with Ruger rings is that they tend to rotate the scope as they are tightened down... tighten slowly from side to side and recheck the scope level as you go.