A flea market find today. Any idea what it was originally for? It's about 3 feet long. or a little more than that.

 

I believe it reads Suzuka Navel Arsenal.  The top set of kanji, which would be the identifier of what was in the box looks to have been worn away.  It was probably a gunsight of some kind.  If you can get a nice clear image of both the ink stamp and top set of characters, Mr. Bushido might be able to identify.  The ink stamp is probably the maker/optics company that made whatever was in the box.

As other members have pointed out, it says "Suzuka Navel Arsenal【鈴鹿海軍工廠】"

I predicted the character on the inside  to be "【碧水】or【碧本】?", so I did some research, but I couldn't find any information about it.

Thanks. It's roughly 44 inches long inside.

Hi , Just been surfing the internet about Suzuka Naval Arsenal and apparently they manufacture ( amongst other items )  three machine guns Type 2   13mm length 60 inches .

Type 3   13mm  length  46 inches , Type 97  7.7 mm  length  40.7 inches .  without any definitive knowledge  may I suggest your box with an inside length of 44 inches ~ could it have held a   :- ( Type 97  7.7 mm length 40.7 inches )  a wild speculation ! Yip I agree but just a thought ~ trying to be helpfull , hope someone with total knowledge will contribute to the article ,   Regards,  Bern .

Hi , The number 1873 on the lid seems to have been painted on at the same time as the kanji characters, does anyone know the significance of the numbers ??

It looks like a British Bren LMG transit case. Missing the internals of course. I’m sure the Japanese used similar cases to store and transport their LMG’s as well. Just my 2 cents.