You will have to try want ads and on line auctions.
You risk damaging the barrels if you do.
You must call Browning with the serial number to find out.
If you have the money....
1972
1986
12 gauge BSS were first made in 1971 and 20 gauge in 1972.
1976
The BSS
1972
50-1000 usd
Made before 1975
1972-1987