This looks really interesting. I thought I had seen all the IJN subs
but this one is new to me. From its small size I'm guessing it's for
coastal defense.
Well, you might have seen all the
Imperial Japanese Navy submarines
... this submarine and her sisters were operated by the
Imperial Japanese Army! They were not warships, and are not armed beyond the 37mm deck gun; they are the mysterious
Maru-Yu class submarine transport. They were used to smuggle goods through dangerous waters to island garrisons... and are very hard to find details on. I know there is an article in Warships International (3rd Qtr of 1999) that has an article on them, but I can find precious little else about them!