I don't know what the numbers mean, but I had a Lanber O/U that was marked 5 and 10. After finding no help to cipher the numbers someone told me that a dime would tell me what I wanted to know.
If the dime will not go in it is full.
If the dime just goes in it is modified.
and so on.
Hope that helps.
I'm assuming it's a 12 gauge.