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 pice of two-coloured heavy wood (lignum vitae)
It has a good aged patina with a few scratches and gouges from use
Dimensions : Height 4½" (11.4cm), Diameter 4" (10cm)
Weight 564g (1lb 4oz)
Ref : Bobbin5
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