A Model 36 with serial number 6444xx was recently identified as shipping in May, 1968. That would give a pretty good indication that yours shipped the same year and probably just a bit later than May. But without a letter, you can't be sure.

1968 would be right on the edge for the diamond stocks, actually pushing the envelope a bit but still possible. Do those number to the gun?

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Jack
SWCA #2475, SWHF #318