The K.IX class was obsolete by the time WW2 started; the only really useful classes were the K.XIV class, O16, O19 and O20, and the O21 class. During the war, two T-class were transferred to the RNethN: Hr.Ms. Zwaardvisch and Hr.Ms. Tijgerhaai. Zwaardvisch saw action in the Far East in 1944-45, but Tijgerhaai was too late to see action- she was proceeding to the Far East when the Emperor signed on the dotted line.
Most numerous was the O21 class, but three were captured by the Germans in May 1940: O25, O26 and O27. Thus, only four saw service with the RNethN during WW2: O21, O22, O23 and O24, O22 being lost in 1940. O19 and O20 were very similar, differing only in their ability to carry 40 mines in addition to the torpedo armament.
The K.XIV class consisted of five subs: K.XIV, K.VX, K.XVI, K.XVII and K.XVIII; O16 was a one-off. Welding was introduced at a fairly early stage, the percentage of the welding in the structure being increased with each successive class- O.16 was 49% welded.
Originally, submarines were built for either service in European waters or service in the NEI, the former receiving the prefix "O" (for "Onderzeeboot" or submarine) followed by an Arabic number and the latter the prefix "K" (for "Koloniën" or Colonies) followed by a Roman number. Eventually this distinction was abandoned and K.XIX and K.XX were renamed O19 and O20 while building. O19 and O20 were the world's first submarines to enter service with a "snuiver", this device also being fitted to the succeeding O21 class
. Later this became famous as "Schnorchel", "snorkel" or "snort". Best would be to model either the K.XIV class, O19 class or O21 class- I have few drawings of the O19 and O21 which I can scan for you.
The "T" class I referred to was specially modelled for SH3 by the GWX team and is a FULL model, not just the conning tower. I;ll post the files on my Filefront page and PM you when done.