This link has a profile photo of a Japanese sub with the deck gun clearly visible. It's a distant shot, but the profile is close.
http://www.history.navy.mil/photos/images/h73000/h73054.jpg
This photo also shows a deck gun that's similar in profile.
http://www.history.navy.mil/photos/images/g330000/g339860.jpg
Another possibility:
http://www.warships1.com/Weapons/WNJAP_47-45_10ns.htm
Hope this helps.
"There's no problem that can't be solved with a suitable application of high explosives"