The M3 washer at the top and bottom is so the bolt doesn’t pull through the rubber washers. You can use a larger rubber washer, and trim the sides so it fits in the slot on the bed.
One on top and bottom act as springs pushing the bed and yoke together, but they have give, allowing the leveling washers to do their thing, and letting the bolts move out of square.
As you increase the angle of the bed, the distance between the bolt head and nut has to get longer you don’t want to overtighten those. Just snug. so the washers can be a spring - if you use the rubber washers and crank it down, it’ll bind