I wondered that myself and thought perhaps they had a numbering system for the size and style of butt plate.

The plates were numbered 0, 1, 2 and 3. And they were molded into the plate.

A 16g gun would typically use a number 1 plate.

Learning, learning, learning. Here's the plate. As you can see it's a 0. I was also surprised to see someone had removed some wood from inside the stock. I assume for weight adjustment. Have you seen this before? It certainly has to weaken the stock somewhat.

Attachment 109153

Attachment 109154

Dan on some of the skeleton steel butt plate guns you can see the round plugs in the butt stock holes.

That boring is likely factory. Maybe not. If it is, it would have been done to balance the gun. Usually when the factory removes wood from under the butt, the holes would be plugged up. Even under a buttplate. But I cannot say that was always the case over the many years of manufacture.

Boring out the butt like that does not weaken the stock at all. It is in the back and does not see any stress. They can be bored out far more than that.
Wood really does not weigh that much. One has to remove a lot from the butt in order to really make any major difference in weight or balance. When it comes to wood in the butt, an ounce or two is about all you can reasonably hope to remove.