2003
1988
BT99
If you go to browning,s home page there is a way to age your shotgun there.
Japan
100-1000 usd
1971
You must call Browning as the sn you gave does not follow normal convention.
100-1000 USD
50-650 usd
Browning doesnt show the BT99 as a 299 model code. They list the BT99 as a model 149. Below is the link to the Browning codes.
Incomplete serial number. Get the complete number, go to Browning's site, Support, Date Your Firearm. They have a complete listing.