The earlier the better. 1 is in the Browning Museum.
You will have to call Browning.
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
You will need to supply the serial numbers to your shotguns to get the age of your browning shotguns correctly.
Comparison to known serial numbers.
This would be a Browning BAR semi-auto rifle
50-500 usd
1975
Check Browning's web site. They have sn data.
The number before the "V"
Under the barrel release lever on the top of the frame