During the definition phase for the Lockheed T2V-1 SeaStar carrier-based trainer version of the T-33 Shooting Star, one T-33 was modified with twin vertical tails as part of program to see if the post-stall stability of the T-33 could be improved. The Navy had concerns about the stability in the carrier landing pattern of a naval adaptation of the T-33.
Aircraft 51-4263 was modified with an enlarged horizontal tailplane with rounded end vertical fins. The rear fuselage tailpipe was also modified and some accounts also note a rudimentary arresting system added to the underside of the fuselage as well. Various shapes and materials were also trialled on this particular aircraft before it was returned to stock T-33 configuration.
The project was never publicized by Lockheed and Naval History Office has no records of this aircraft. Sometimes called NT-33, according to two individuals who worked on it it had no formal designation. The twin tail configuration was never adopted for the SeaStar as Lockheed elected to enlarge the vertical fin and move the horizontal tailplane further up the fin on the production T2V-1.
Source: Wings of Fame, Volume 16. AIRtime Publishing/Aerospace Publishing, 1999, "T-Birds and Silver Stars: Lockheed T-33/T2V Variants by Robert F. Dorr, p111-112.