Congrats on your purchase.
Was that serial number all on one line with no spaces? Like this "80R4073". Then I think it's made after 2000 and I don't know what the "R" means.
Or was it written;
I have a Armstrong Model 90 written
It also says" Sterling Body
And Head Only"
written about 19mm lower on the body.
If it was written out on different lines, and with spaces as above, it is a model 80, and the the "R" means it was made in 1970.
The numbers on the head joint "073" indicate that it is the original head joint that came with that flute when new.
My Armstrong does not have the original head joint. So the Head joint does not have matching numbers, or any numbers.
I have some 1950's Artley's that look just like Armstrong's, and also have the last three digits of the Serial # stamped on the Head joint.