Quoting the museum placard:

From August 1968 until early 1973, this T-39A Sabreliner was assigned to Bergstron AFB, Texas, in support of former President Lyndon B. Johnson…

Following President Johnson’s death in January 1973, this aircraft flew as a test bed for advanced avionics and communication equipment…

This T-39A continued to serve as an executive transport for many government and Air Force officials until its retirement to the Museum in October 1984.