The Navy wedded a greatly modified Tang class hull type to the specialized needs of the next generation of Regulus missiles, the Regulus II. The bow of the two boats thus modified was lifted to accommodate four Regulus Its each. They were the Growler (SSG-577) and the Grayback, (SSG-574). By mid-1958, USS Grayback (SSG-574) and USS Growler (SSG-577) had been commissioned as the first purpose-built Regulus submarines, each carrying two in a large bow hangar. At that time, the Navy had four SSGs and four missile-carrying cruisers at sea.
Each of these two near-sister ships - displacing approximately 3,600 tons submerged - could accommodate a total of four Regulus I missiles in a pair of cylindrical hangars set into the large, bulbous bow. These hangars opened aft through a set of doors by which the weapons could be moved onto a trainable launch ramp set into a well forward of the sail. The ramp was rotated athwartships for launching. Growler had a similar outward appearance to Grayback but was quite different in her internal layout. Aft of the missile guidance center Growler's layout was nearly identical to the smaller USS Darter (SS-576).
Regulus deterrence was maintained in the western Pacific until May, 1964, when Halibut conducted the final patrol of the series. By that time, the five Regulus boats had conducted a total of 40 WESTPAC deterrent patrols since October 1959 - and in so doing had pioneered one of the central strategic paradigms of the Cold War.
With her Regulus installation removed, Grayback served as an amphibious transport (LPSS-574) from May 1969 to mid-1980. The ship was later stricken from the Navy list in January 1984 and sunk as a missile target in 1986. With her missile handling and guidance equipment removed, Halibut was converted to a test platform circa 1965 and used ostensibly in developing the Deep Submergence Rescue Vehicle (DSRV) - but actually for more highly-classified projects - until she was decommissioned in June 1976.
The happiest fate was reserved for Growler, which was decommissioned and placed in reserve in May 1964. Stricken from the Navy list in August 1980, Growler is now preserved in virtually original condition as part of the USS Intrepid Sea-Air-Space Museum in New York City, along with an example of the Regulus I missile.