The head of each dog is just a shaped block of plywood that’s counterbored to accept a short length of dowel. The shallow “V” allows the head to grip round workpieces, while the dowel fits into the dog holes of your bench.
A hex bolt runs through the head and the length of the dowel with a counterbore in the top. A washer and locknut thread on the bottom to hold everything together.
When all of the parts are assembled, it makes for a quick and easy way to get a grip on all of your round and odd shaped workpieces.