All good ruger M77s are milled to take "Ruger Rings" they are a proprietary design. There were a few early production that were milling errors which escaped without the transverse cuts for the aligning bar on the bottom of the scope rings. You have some kind of adapter base bolted to the receiver.
Think REAL HARD before you move away from Ruger Rings. Ruger Rings SEEM like a pain in the neck, until you see the advantages.
Any decent gun shop will stock them, despite them being less common than picatinny rings.
The ADVANTAGE to ruger rings are:
Sleek
Low profile
VERY STURDY
Very stable to temperature changes
Very impact resistant
The uncanny ability to unbolt the ring bases, remove the scope with rings on, and reinstall the ring bases without losing scope zero. It will be very close if you are consistent with torque.
Once you have several of these rifles, and have different sizes of these rings on hand they are pretty awesome. I also like having the top of the receiver open, it gives more room for my fingers than reaching under a rail.
Embrace the ruger rings.