According to McDonnel Douglas the original designator for the F-4 was
FB-4.
A designator is used to identify the mission of the Air Craft. Due to
the
Salt Treaties, the B designator had to be dropped as well as the B
designator from the F-111. Wit

Not sure what the original designator of the Navy F-4 was. Could have been
FB-4. Did the Navy have a nuclear role for the plane?

The first AF version was an F-4B the AF called the F-110. Nearly all theF-4
wings in Europe sat nuke alert with real silver bullets.