It's a shot measuring device, that counts individual pellets. Pour some shot on, level it off and what remains in the dimples is your shot charge. On the bottom it looks like it's graduated for different pellet counts. Without knowing scale, it looks like it might be used for larger pellet sizes.

I think there is a drawing of a similar one in Greener's "The Gun and it's Development"

“I left long before daylight, alone but not lonely.”~Gordon Macquarrie