You can start your search at browning themselves,if you have no luck you may want to go to numrich gun parts corp.
12 gauge BSS were first made in 1971 and 20 gauge in 1972.
This is a Browning Side by Side shotgun (Double Barrel) for a 20 Gauge shotshell. The production of the BSS side-by-side shotgun began in 1978. The Sidelock model was introduced in 1983. Production ceased in 1987.
A Browning side by side 20 gauge shotgun in Grade I,with a original finish of between 70%-90% will bring between 1,000-1,800 dollars.
If you have the money....
1972
1976
You must call Browning with the serial number to find out.
It was made in 1981.
You risk damaging the barrels if you do.
1973
1980
50-750 usd