Michael,
After looking at the dimensions of the Stevens action and at Leopold’s stature in the first image, I would agree that the cartridges are more likely 28-35 Stevens - relatively larger, straighter and little taper, with a seemingly larger rim.
As you well know, these old boys were fierce offhand competitors, and relentless experimenters in service of that competition off the bench. But when they wanted to just have some fun and let their hair down, they seemed to like to go hunting for ground hogs, squirrels, grouse, and such. I can’t recall exactly in which book, but either Pope or Mann writes about the two of them just getting away to hunt ground hogs. Maybe it was Mann trying to coax or entice an exhausted, overworked Pope away from the shop for some outdoor recreation.
Bob
