Tatum's Cast Iron What? Ink well holder? Caster? Pin holder?

Bought this in a box-lot at an auction. It is made of cast iron, 3" in diameter and 1 1/16" high. The well is spherical. There is a Coats & Clark's thread label in the well but I think someone may have just glued it there. The item was Japanned and about 70% remains. The wear and chipping to the Japanning is more or less even throughout (but little in the well) and gives no indication of any particular pattern. I've looked at casters, ink well bases, pin holders, etc. with no luck. Tatum made everything from ink wells to suppository makers so it could be anything.