Welcome from TN! If you could ask your question in the appropriate forum, preferably with some pics, I'm sure someone could give you the answers. The 15K followed by three digits is kinda strange. At least from what I see in the book. The 15-3 came out in '67, and in '68 the diamond around the stock screw was eliminated. If yours doesn't have diamond stocks, it would narrow it down to 1968-1977, when the dash 4 came out. The other numbers you mention are probably inspection numbers, etc.

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Misty