Look (or have the seller look) above the serial number for a model number. Or under the Armstrong logo.
Otherwise, have you checked online? I know there's an Armstrong database out there somewhere. If you can't find the model on the flute, I'll try to re-find that site. Might be a little waitthough as I've got busy filling my next few days.
Flutes:1975 Gemeinhardt M2 in chrome nickel;1982 Armstrong 80;2006 Yamaha 584
Piccs:1978 Artley piece of crap 15 P;1982 Gemeinhardt 4S;1980s? Armstrong all wood (no model)
Bass:2006 Jupiter di Medici G0199
Treble:2009 Guo New Voice
+ many flute-cousins