The bt 99 seril # is read as follows if one letter it is an early one like : 2xx d 8 . It was made in 1968.
Later they included two letters and the following number is the year made.
If you go to browning,s home page there is a way to age your shotgun there.
2003
1977
1993
1990.
1991
12 gauge Belgian Browning shotgun, serial #296305
You will have to call Browning or provide the serial number.
With the serial number that you provided,your Browning superposed shotgun was made in Belguim for Browning in the year 1955.
This Browning shotgun you describe was made in 1959.
You will need to call Browning.
You will need to call Browning.