Machinist here
This all looks custom for the product
While I will admit I have seen nuts and washers akin to those in the photo, it’s clear to me that the hardware is custom made based on the finishes presented on the face of the flat bolt.
The bolts’ rotation is being held still by ‘grips’ or ‘teeth’ on the bottom side of the bolt head and to the shoulder of the mating face with the washer. The washer is also rotationally helping the bolt stay stationary, with similar teeth features. Beyond all this, the faces of the blades are simply ground flat, so even without a bearing shim or other bearing mechanism, the blades will slide smoothly past one another.
See tooth lock washers on McMaster And plow bolts on McMaster
Again it’s going to be hard to find any product that directly represents what you see In the photo.