Not gun related at all. This jig attaches to the mill bed and allows you to angle the piston to perfectly align it relative to the valve angle prior to doing the plunge cuts for the valve reliefs.
This especially important on canted valve engines like the BBC's and on any situation where the heads have been angle cut. This keeps you out of trouble with the radial clearance. There are different drawbars for different pin diamters...you can see one in the background.
