They are for a K-frame. From a picture of the front side, it is difficult to tell K grips from N grips, but no problem if you look at the back side. The (C)305877 SN suggests 1954-55. There can be no mistaking the visual differences between pre-war and postwar Magnas.

Did you buy them for an existing gun? Similar grips are VERY common to find on eBay, in the $20-$40 range depending upon condition.