I know nothing about horses, except when ever I ride one, I am always behind another horse that seems to have eaten beans for breakfast. I think Cudamark is correct. A frame for a magnifying glass.
I think we need to know if there is a groove on the inside of the hole that a piece of glass or mirror would fit into. (I believe clock pendulum weights were adjustable to move up and down on the vertical piece, but don't know how I know that.)
One thing that I can be very clear on: it's frigging awesome!! - nicely detailed, great for display, and so far, a mystery!