0
You will have to call FN and provide the serial number.
Your browning model auto-5 shotgun was made by FN for browning in 1949.
Fn = Fn-1 + Fn-2
And F0 = F1 = 1
The first number, f1 = 1
The second number, f2 = 1
After that the sequence is defined recursively: fn = fn-1 + fn-2 for n=3, 4, 5, ...