At first I thought it was one of keltos01's early attempts at making a playable Japanese Jyunsen B submarine. However, Ducimus answers the question in this thread.
http://www.subsim.com/radioroom/showthread.php?t=137201
It is a modified conning tower on a Type9 hull done by Natilus42. While resembling some of the Japanese conning towers, it does not match any Japanese I-boat conning tower. Closest matches would be I-61, I-65, or I-68; which are Kaidai Type 4, type 5, and Type 6A respectively.
See:
http://www.combinedfleet.com/type_kd4.htm
http://www.combinedfleet.com/type_kd5.htm
http://www.combinedfleet.com/type_kd6.htm