Hi - Armstrong's alto flutes in the Heritage line were/are numbered asmodel 703, but typically have a plated head joint. It's quite possible someone decided to upgrade to sterling for the iimprovement in tone... lucky you
Have you looked for the serial number on the body? Try looking around the left hand keys. I have seen older models with the serial number tucked in there.
I have a vintage 1977 sterling Armstrong 80B that I bought new as a 10th grader and still play. With care and maintenance, Armstrong's seem to last forever.
I hope this helps. My other suggestion is to contact Armstrong in Elkhart,IN. They have helped me with a few mystery instruments.