Roller bolt

the bar in a carriage to which the traces are attached; a whiffletree.

See also: Roller

Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.
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So different have the two halves of the the last two campaigns been that they resemble two cut-and-shut motors with half a Roller bolted on the back of a Reliant Robin.