The four pieces that made up the attach plate are already match drill (undersize #40) . The goal of this operation is to match drill the whole assembly to the web and the front and back capstrips. In general the capstrips are match drilled already. In the area under the attach plate however, there are no holes until this operation is completed.
The drilling jig is a cheap vice grip that has been cut up and attached to a floating drill bushing. The drill busing is a 1/2-13 bolt that has been center drilled with the drill size to be used. The attach plates had a #40 match drill already. The pin's tip is sized for a #40. The pin's shank is 1/8. By placing the pin in the #40 attach plate hole, and then locking pliers down, a larger concentric hole can be made thru entire assembly. I use a 1/2-13 bolt as the bushing. It can be swapped out with whatever size drill bushing I need.
The advantage of this clamping fixture in addition to assuring concentricity is that there will be no metal shavings between plates after drilling.