To find the year of proof of your (Post 1975) Browning or Miroku shotgun you’ll need to find the Serial Number. On most over and under shotguns this should be situated under the top lever…
and on Semi-Autos somewhere on the receiver...
In the Serial number there are two letters together - in the example above this is ZT - this your date code. If you don't have this it is likely that your Gun is an earlier model.