These standard, short mounting plates, attach to the chassis or other structural member and provide an anchor point for the Dzus fastener.
They're individually mounted on a soft rubber backing, so no cutting is required (and no lost parts).
Photo of real world mounting plate. Plates mounted on chassis.
Example of plates on Clay Kemp's 1/16 Mustang
With 60 pieces included, there are enough plates here for a few cars.
See also the Dzus quarter-turn fasteners that would attach on the outside of the body panel.
One set, photo-etched .006" nickel-silver plate, by Futurattraction.