a stands for 12g g stands for gamster
It is the rifling in a mossberg rifle barrel.
Looks like the serial number to me.
a is for 12ga and the g is for gamester
Mossberg
Moss Mossberg's birth name is Richard Mossberg.
You will have to contact Mossberg
Mossberg 500AB
at the mossberg firearm factory.
If it is a Mossberg, it was made by Mossberg. They did make guns for a number of "store brands".
You will have to call Mossberg
It means you have a Mossberg Model 500a shotgun with the serial number L109563.