Here is something for sale on ebay in the US. They give its dimensions as 3 X 16 inches. I do not recognize the markings, but I thought some of you might find it interesting.
Yes - "8CM Type 3 AA gun" or "8cm 40 caliber dual-purpose gun". Mine measures 76.2 x 409.
The Japanese produced about 60 guns a year from 1931 -1945. Very common on the islands in the pacific. It had HE as well as APHE rounds available for surface targets.
Incidentally this case was reportedly taken on Guadal Canal. One of these days I'll have to do some research and verify these guns were there. Since there was an airfield under construction they would have had some air defense.
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