Only photo I could find of the O.21 on line. Notice the opening for the traversing torpedo tubes forward of the conning tower.
The book I use,
Submarines of World War II by Erminio Bagnasco was published by the Naval Institue Press bt is out of print. Copies are still available via the Internet.
http://www.alibris.com/search/books/...ld%20War%20Two
http://search.barnesandnoble.com/Sub.../9780870219627
