Hi Dave,
Finding an aircraft's history in the US Army, is the hardest task out of the three services. I have seen pages from something called "Goldbook" which list an aircrafts unit allocation by each month of its military service.
Gary Roush at the Vietnam Helicopter Pilots Association may be a good contact, as to where you can find the history of your helicopter, but you will need to provide him with it's military serial number. Do you know the military serial number? From that I can give you the Hiller construction number.
Gary has done a lot of work on the individual histories of US Army aircraft in Vietnam. Try contacting him at
webmaster@vhpa.org
Regards
Sid