As mentioned above, any good 3 screw OM is a good find. If the action hasn't been converted to the transfer bar system, a big plus. As for the brass "backstrap." As mentioned, if it will "letter" (meaning that if you contact Ruger Records, ask about how it was shipped, supply them with the serial number, and they come back and say; BKH-36B, then it was shipped with the factory brass gripframe,) then it's very valuable. If it has a factory brass gripframe that was ADDED after it was shipped as a BKH-36, (notice the missing "B") then it still has good value. A factory brass gripframe can be distinguished by the frame markings, "MR3-DB" under the grip panels. A factory brass gripframe by itself is desirable. If it's an aftermarket brass gripframe, then, it's just a good looking shooter! Still worth having, but not as valuable as previously mentioned.
The devil is in the details.