The "washer" below is from a carriage bolt. It's like a t-nut, but used on the square side of a carriage bolt and holds it steady. I'd love to get my hands on a box of these but can't for the life of me Google the correct terms. Hopefully someone here will be able to point me in the right direction.
For what it's worth, the "washer" came from an older kids playset that was custom built.
