It looks as if it is supposed to have a fairly large thick washer, and all 4 bolts should to through it as well as the disk. The washer then supports the disk and keeps it from getting bent and running out of true. I can't tell at present whether the washer should have a small central hole so that the big nut holds it as well as the 4 bolts, or it would be a clearance hole for the big nut. However if you are going to buy a new disk, I recommend that you buy the washer at the same time. If you use the real part, there aren't as many unexpected outcomes.
