George;
I have lived in Lake County California for nearly all of my 63 years. I have had the good fortune to have seen, and handled, many of Charlie's fine rifles, and not a few of those made by other members of his family, But, I have never seen a Slotterbeck drilling. I was told years ago that one was for sale at Friendship, that was a 16x16x38. I have never seen a Slotterbeck with a capbox either. Although I own a rifle with another makers name on the barrel that legend states was made in Charlie's shop. Was the capbox by any chance a commercial iron box, with two screw mounting? This rifle has such a box, mounted reverse as you indicate.
Hungry Horse