An early 1800's rigger's serving mallet used to wrap marline around a splice in a rope.. The groove in the head fits around the splice and a turn of malice is taken around the handle, friction holds it tight. The tool is then rotated around the rope.
This one measures 16.5cm l x 6cm w at the widest point x 1.6cm d 
 
 
For more info on antique treen sailmakers tools  see link below https://www.opusantiques.co.uk/single-post/2019/07/11/Antique-Treen-Sailmaker’s-Liners