These are available in three sizes, to suit 3/4″, 7/8″ and 1″ hole sizes.
Offset Prickers are used to mark Tenons through pre-drilled Mortice holes in Traditional Timber Frame joints.
The tenon is then removed, and drilled where the mark indicates: 4mm closer to the shoulder than the centre of the peg hole in the mortice.
When the joint is re-assembled and the peg driven in, the offset of the holes will cause the shoulder to be forced snugly against the face of the other timber, tightening the joint.
Ive been supplying these since 2001, when the design was suggested to me by Steve Lawrence and the late Jake Jackson.