This is a Georgian sail makers tool known as a liner or rubber made from one piece of wood.It was used for smoothing the seams in canvas. It was achieved by pressing and rubbing  the waxed stitchesinto the sail canvas.
This example is chip-carved with initials, dated 1801. It has carved foliage a carved diamond and a multi-faceted handle with carved pattern.
It measures 11cm l x 3.5cm w.