The six submarines of the Tambor-class were authorized for construction in Fiscal Year 1939, and the six boats of the Gar-class in 1940. Other than that the 12 boats are of the same design, with numerous small differences depending on who built them (government navy yards vs. Electric Boat). There is a great deal of debate amongst historians as to what class these 12 boats fall into. One historian (Alden) considers them to be one class, the Tambors. Another (Friedman) considers them to be two classes, Tambor and Gar. Strangely, official Navy documents are not consistent in this regard and this adds to the confusion. Personally, I have adopted the convention of referring to them as the Tambor/Gar-class. For further information on this pathfinder group of boats, see the postscript in the article at this link:
http://navsource.org/archives/08/pdf/0829294.pdf