The Pursuit designation was only used by the Army
Air Forces. The Navy used a different designation based upon letters
designating their different missions with F meaning fighter. From a book
called U.S. Fighters, "In 1962 the Defense Department took a step to
eliminate the conflicting designations by the Air Force and Navy when
referring to the same aircraft. The Air Force, upon acquiring the McDonnell
Phantom II, identified it as the F-110A, while to the Navy it was the F4H-1.
Under the 1962 system, the Phantom II became the F-4, a single letter after
the "-4" denoting which version." That numbering system continues today.
Experimental and production aircraft continued to receive sequential numbers
using this system. However, the Air Force continued to use the old system
for some special classified aircraft. For example, the Air Force received
some Soviet aircraft and gave them a "cover" designation using the old
system to try to hide their true identity. Since the F-117 was in the
classified arena for a long time, the Air Force again used the old system so
that no one would question why a number was skipped under the new system.