Hi,
Try the Millet rings made for Ruger rifles. These come in sets with one #4 and one #5, suitable for the 77MK 11 rifles (one lower, one higher to match the profile of the action on the rifle). These also have the small ridge that fit into the groove on the top of the action. Not ssure which model rifle you have.
Unlike the original Ruger rings, these are adjustable / can be moved for windage adjustment, but this also means that if you take them off, you cannot put them back and still have more or less the same POI (point of impact) as is the case with my Ruger rings. The zero will have moved.
I bought mine from Epps (only sets of medium hight and in nickel finish in 30mm) about 6-8 weeks ago. They had several spare sets in the store. PM me if you need the exact product number.
If your rifle has a rail as on the Gunsite Scout, or a level (same hight) as on some single shot #1 rifles, try the QD Leupold 30mm rings. I prefer these even over the original Ruger rings.
Hope this helps
RSA1