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Type 91 Naval Anti-Aircraft Mechanical Time Fuze

The Type 89 12,7cm (5-Inch) twin tube anti-aircraft gun was the standard AA gun of the large battleships and aircraft carriers of Imperial Japanese Navy during WWII.
Originally designed for use on ships, the Type 89 was also placed in dual mounts at land installations.

Imperial Japanese Navy aircraft carriers Zuikaku (foreground) and Kaga (background) head for Pearl Harbor, November 1941
Type 89 AA Gun Mounts visible in foreground.

This is the Type 91 Mechanical Time Fuze, used on ammunition for the 12,7cm AA-gun. Versions were modified with a gear ring for use with gun mounts equipped with an automatic time setting device. At right is a Type 91 without the gear ring.
The body is made of nickle plated brass, with an aluminum nose cap. The time setting dial is inscribed from 0 to 55 seconds, in one second increments.

Images of lables found on the lid and on the safety wire. The left lid label appears to be dated January 1944.

While Army fuzes seem to have all kinds of useful data stamped on them, Naval fuzes seem to have very little.
Here we can see "1-" followed by the serial number (also stamped on the gear) and what I assume to be the date "Showa 16-9" Sept 1941.

To the right is the Kana Character "To" followed by an anchor. A mark for the Naval Arsenal that made the fuze, possibly the Toyokawa Navy Yard? Just a guess here.

Email conversations have brought an observation to light that's worth mentioning.
Seems there are Western style three letter codes, observed on some Type 91 fuzes, both with and without the timing gear ring.
So far I've only seen these on Type 91 fuzes. They seem out of place, considering the usual Kanji one would expect. Post-war inspection stamps perhaps?
At any rate, here are some photos. Note the following codes: "AHJ","AMT", "ARD", "ARK","CMA", and "DPO"
Anyone have some info as to what these indicate?

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