Early war conning tower. Its acutally historically accurate. If you dig around here:
http://www.navsource.org/archives/08/04idx.htm
You'll find all sorts of pictures. Later on you'll get a refit (happends randomly when returning from patrol) which cuts the weather fairings away. Or you could always mod it and force the midwar tower earlier.
For the most part, the war progression on US subs looks similar to this:
http://www.ducimus.net/sh4/release/porpoise_refits.jpg
http://www.ducimus.net/sh4/release/s...rgo_refits.jpg
http://www.ducimus.net/sh4/release/T...gar_refits.jpg
http://www.ducimus.net/sh4/release/gato_refits.jpg