Spent some time putting in the lightening holes in the seat backs today. First, I opened up the starter holes to 3/8" to accomodate the bolt used in the 1.5" diameter knockout punch.

Here are all the punched out pieces. Saves about 140 g or 5 oz of weight.

Deburred with a hand deburring tool and smoothed all the edges with a 1" diameter scotchbrite wheel.