The recent sinking of a South Korean warship Cheonan has brought more details of DPRK's midget submarine fleet into the public domain. Reliable information about the "Yeoneo" class submarine comes from captured Sang-O crewman Lee Kwang Soo, interviewed by South Korean blog Daily-NK ( http://www.dailynk.com/english/read.php?cataId=nk02500&num=6445 ).
Based on his information, I've updated my sketch of the most probable North Korean midget submarine class involved in the incident. The externally mounted torpedoes are described by the former DPRK submariner. Lee cites quite launch, whereby the torpedo 'swims' away under its own power rather than being forcible ejected from the torpedo tube (which is noisy and may alert the target to the launch) as the motive for this arrangement. This is plausible of course and is not unique among small submarines. The disadvantages of course are performance and weapons maintenance.
It should be noted that the previously captured 'P-4' boat clearly had design adaptions for two internally mounted torpedo tubes, and the essentially similar IS-120 'Ghadir' class operated by Iran also has internally mounted tubes.