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
1990.
1993
1991
You will have to call Browning or provide the serial number.
12 gauge Belgian Browning shotgun, serial #296305
With the serial number that you provided,your Browning superposed shotgun was made in Belguim for Browning in the year 1955.
2003
1988
Your browning 20ga model auto-5 shotgun was made by FN of belguim for browning in 1962,with the serial number provided.