14 C.F.R. § 61.7
Obsolete certificates and ratings
(a) The holder of a free-balloon pilot certificate issued before November 1, 1973, may not exercise the privileges of that certificate.
(b) The holder of a pilot certificate that bears any of the following category ratings without an associated class rating may not exercise the privileges of that category rating:
(1) Rotorcraft.
(2) Lighter-than-air.
(3) Helicopter.
(4) Autogyro.