The original rears are 5/16" coarse thread bolts about an 1 1/4" in length with fairly large star like washers. These washers are affixed to the shank of the bolts and are slightly "mushroomed" so as to pinch the sheet metal tightly.

The washers are roughly an inch or so in diameter . All but two of mine were gone, too,
so I just used new 5/16 bolts and flat washers .... worked fine.
mike