A 4" barrel version would have been a .22 Combat Masterpiece, and the 6" the K-22 Masterpiece. Model numbers would have started in about 1958, and somewhere in the early K 300000 serial number range.
Now the mandatory photo posting, please . Hope this is helpful.
Yes. A whole bunch of K152xxx range guns shipped in October of that year. So early to mid is likely, but not certain.
But I'm curious: how did you conclude that it had to be a K-22 Masterpiece and not a .22 Combat Masterpiece based just on the serial number? Was the K133xxx range blocked out for the K-22?