Well, Here's my take on it.
The first 12 Connie Cruisers were similar to the Enterprise. When Kirk said there was twelve like them in the fleet (to Captain Christopher), it meant that there was twelve Constitution Class Cruisers at the time. Which would be 1071 Constellation, 1371 Republic, 1700 Constitution, 1701 Enterprise, 1703 Lexington, 1704 Yorktown, 1705 Excalibur, 1706 Exeter, 1707 Hood, 1710 Kongo, 1711 Potemkin, theres another one but I can't find/nor remember the hull number and name.
The next set was 1712 Bonhomme Richard, which while still looking like a Constitution, had some subtle difference that may or may not have included rear firing torpedoes. 1764 Defiant obviously belongs to this subclass.
(Of course, MY take on it is going to be completely thrown out most likely when TOS remastered gets to The Omega Glory or The Ulitmate Computer)