Another antique item from yesterday

This old Ideal tang sight, missing the stem. It is the type where the stem is smooth except for a thin threading on the front. Patent dates are 1905, 07. It has three click locking positions in the upright. What’s unique is a single mounting hole. I wonder what it fits?!

I can’t find a letter code on the base anywhere. Maybe I don’t know where to look. It’s interesting though.