The last 2 letters in your serial number NZ indicate that your Browning 325 in 20 gauge was made for browning in the year 1991.
This gun was made in 1947
see the Browning website
1980
1973
1929
You will have to call Browning.
This gun was made in 1950.
1931
1950
It was made in 1952
Your Browning superposed,over under 20 gauge shotgun was made in the year 1951;with the serial number that you have provided.
Browning.com has a sn function under customer service