Who understands and can explain theses numbers and what they mean?

As the title states, can anyone explain what there’s numbers mean? And if you have a diagram to demonstrate what the numbers mean and where on the barrel they are? I have no clue and cannot find anything on the internet for it. Thanks!

As the title states, can anyone explain what there’s numbers mean? And if you have a diagram to demonstrate what the numbers mean and where on the barrel they are? I have no clue and cannot find anything on the internet for it. Thanks!