[CapnScurvy]

For the real life subs used during WWII the various class's usually started off with the particular subs "Commissioned" date. Which is different from its Sea Trials Dates, or its beginning of existence date that's referred to its date of Laying its Keel (starting construction). The Commissioned date is when it actually went into service.

Here's the general progression of the various sub classes that many times had their name given to the class, and their initial boat ID's:

S-Class............ ID.........Commissioned
S-18.............SS-123......April 3, 1924
S-42.............SS-153......November 20, 1924

Narwhal
USS Narwhal...SS-167......May 15, 1930

Porpoise
USS Porpoise...SS-172.....August 15, 1935

Salmon
USS Salmon.....SS-182.....March 15, 1938

Salmon
USS Sargo.......SS-188.....February 7, 1939

Tambor
USS Tambor.....SS-198.....June 3, 1940

Gar
USS Gar...........SS-206.....April 14, 1941

Gato
USS Gato.........SS-212......December 31, 1941

Balao
USS Balao.........SS-285.....February 4, 1943

Tench
USS Tench........SS-417.....October 6, 1944

Note: These subs can be found listing their "Commissioned" dates differently....depending on which resource you choose!?! But, this will give you the general idea. The stock game lists some of their "Start Dates" differently too (as do almost every mod that's tried to look up these specifics). So, use this as a general rule.