https://www.adams-music.com/en/repair_and_maintenance/serial_numbers/wt_armstrong says,"From 1974 to present - The prefix number plus 50 will give you the manufacturing date." That gives you a manufacture date of 2024 (+30=2004, +40=2014, +50=2024), which, if you're looking at obviously older flutes, is wrong.
Other serial number lists I've seen stop at 1974.
One thing that a lot of clarinet manufacturers do is that they have one list of serial numbers for your standard professional Bb clarinet and a different list for "harmony" instruments, like a bass clarinet. I have no idea if Armstrong does that with alto flutes vs. standard C flutes.
We've got bunches of folks here that know more than me, so you might get an answer in a few minutes

. Other than that, the best suggestion I have is to contact Armstrong.