Here's an NOS set of Delco J56 heavy duty brake pads. The full set of four is part number 5468882, while apparently each individual
pad is labeled as part number 5468883. The set includes the four necessary cotter pins in an envelope, p/n 5474322 and an instruction
sheet indicating how to precondition them for the best braking performance. The backing plates on these pads are made with Inconel,
an expensive, high-temp alloy that was more resistant to warpage when they got hot, unlike steel. Inconel is a non-magnetic material.
The thickness from the back of the backing plate to the front surface of an NOS brake pad is 1/2" (.500"). (Jon Mello Collection)
The cotter pins are around 4" long and are 1/8" (.0125") in diameter.
Instruction sheet p/n 5474920
Warning: Preconditioning of this material is necessary to
obtain maximum effectiveness. After relining the brakes,
drive the car for approximately 1/2 mile at 1/2 throttle
with the brakes held on enough to maintain a car speed
of 15 to 20 MPH.