That is a very nice older .22 Combat Masterpiece, made during the relatively short duration of Model 18 production (1958-1959). As you indicated, it probably shipped in 1960.

The grip adapter is, as banger noted, a Pachmayr product. These began life as a Mershon product and were called the Mershon Sure-Grip. Mershon was a California company that was located in various places in Southern California, lastly in Glendale. I have examples of them in my collection with different addresses on them. I have not yet determined the several dates when the address changes were made, but it makes for an interesting study.

Eventually, Mershon was bought out by Pachmayr, sometime in the late 1950s. I can't read the lettering on yours in the picture, but it should say Pachmayr on it, along with an address and some dates. It is probably contemporary with the gun.

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Jack
SWCA #2475, SWHF #318