I wrote a short blog (here) about these manger balls / chogs a while ago.
They are variously known as chogs, sliders, manger balls, manger logs and tying up bobbins.
They were traditionally used to weight a rope that passed through a ring or slot in a manger and attached to the halter or headcollar, thus allowing the horse to move but preventing the rope from looping dangerously.
This example is turned from a single piece of very heavy wood or lignum vitae
It is smooth and almost black with a few flashes of gold coloured wood.
It has a good aged patina with a few scratches and markss from use.
Dimensions : Height 3¼" (8.5cm), Diameter 3¼" (8.5cm)
Weight 516g (1lb 2¼oz)
Ref : Bobbin008
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