I don't know anything about codes. The Olin Corp. kept the Winchester and Western lines separate through the 1974 catalog. In the 1975 catalog we begin seeing the Winchester-Western on the ammunition boxes. By 1982, Olin Corp. dumped the struggling U.S. gun business off on the employees as U.S. Repeating Arms but kept the Winchester name and in their 1982 ammo catalog the boxes are all marked just Winchester.
Always seemed wrong to me as Western Cartridge Co. was their original business and I feel Western was the better ammo in the 1920s. Western introduced progressive burning powder, high velocity shotgun ammo with their Super-X loads in 1922.
Just seems horribly wrong to me to see a Super-X box with the Winchester name on it.
The Olins bought the defunct Winchester Repeating Arms Co. out of receivership at the end of 1931. Within a couple of years, the Winchester ammo line came to mirror the Western ammo line.
To get a ballpark, consider the method of print. The rudimentary print features on this box 'belong' somewhere in the general history of the company and of the industry in general. Not foolproof. But you could wag it.
Your box is labeled "Winchester-Western Shotgun Shells". Although this is seen first in the 1975 catalog, it is logical that it first appeared on boxes in 1974 but after the 1974 catalog was printed. This box design was changed to the white box with the orange-red banner including "Winchester" and the Pony Express rider about 1982 or 1983. So the box design tells you that your shells were made between 1974-1982.
As to the lot number, fortunately I found a better source, by d'Artagnan on the mdshooters forum in 2015, than I had used in past years. Your lot number inside the top box flap is Y3HM1CH, so the day code is 1 and reading leftward we isolate "HM1". "H" = 1934, 1954, 1974, 1994, or 2014. (I got inconsistencies with my older source H = 1942. 1966, or 1990.) "M" = November.
Your shells were manufactured on 01 Nov 1974, sir.
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