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