The QT-2PC night reconnaisance aircraft (a tri-service DARPA/MACV) evaluation used "Hawk" or "Night hawk" in the first quarter of 1968 and perhaps later starting in April 1968 (probably only three months).
The follow-on version (YO-3A) probably used a similar call sign from 1970 to 1972.
Check with USA 221 RAC ('68) and USA 73rd and 131st SAC and 220 RAC ('70).

Dale Ross Stith