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.
Please add the serial, but it is likely a single barrel trap gun such as the BT99
If you go to browning,s home page there is a way to age your shotgun there.
BT99
100-1000 USD depending on condtion and EXACT configuration, papers, box, accessories.
Japan
A shotgun designed to shoo trap with.
100-1000 usd
1988
1971
2003
You must call Browning as the sn you gave does not follow normal convention.
100-1000 USD