Hi Rich,
Nice round!
The headstamp is straightforward, the case looks like a 75x294R case and it was made at Nagoya arsenal, in December 1937. I'll explain how to read the headstamp: The '12' represents "month 12" in Arabic numbers, i.e. December, and the '=+' is Kanji for '2' and '10', hence forming '12', followed by the Kanji to indicate that this is a Showa date, meaning that the 12 is relative to the base of 1925, i.e. 1925 + 12 = 1937.
The arsenal can be read from two bits of information, being the arsenal mark (at the 6 o'clock position), in this case a big circle, with two smaller ones on top of one another on the inside, being Nagoya. It is also reflected in a single Kanji, appearing around the 8 o'clock position. Often, but not always, one finds a single Katakana symbol around the 7 o'clock position, which I think (but I'm not certain) may be an acceptance mark or so. Also, not shown in your image, but typical on Japanese army cases (such as this one) is the letter 'F' at the 12 o'clock position.
Though you didn't ask for it, I can also explain (most) of the markings on the projectile. It was made at the Osaka arsenal (reflected by the crossed cannons, and a single Kanji, being the second to the left of the crossed cannons (I'm not quite certain what the symbol is that appears in between those two symbols), in February of Showa year 17, i.e. 1942.
Cheers!
Olafo