Yes, I concur with weaselmeatgravy. At some point Ruger ran out of flattop parts, so finished the cylinder frames frames up with what they had. I wouldn't worry too much about the serial number... that is still a very late serial number for a flattop, so not unlikely it may have shipped late.
However, the medallions look like they are post 1966. And the trigger is super thick, like it is from a gun of at least the late 1960's.
Is the grip frame ALCOA G or ALCOA G2? With this info, a ship date will tell you if these parts are correct or not.
Chet15