You will have to call Browning.
You will need to supply the serial numbers to your shotguns to get the age of your browning shotguns correctly.
Check Browning's web site. They have sn data.
Blue Book of Gun Values
Try Browning.com
Browning.com has sn data under customer service.
The earlier the better. 1 is in the Browning Museum.
The suffix of M71 was assigned to the Browning model BAR(Browning semi-auto rifle) which was made in the year 1971 in Belgium for Browning.
All Brownings have serial numbers.
1981
Browning's web site has a sn search function.
Browning never used the letter B prefix for any of there shotguns serial numbers.
Comparison to known serial numbers.