I recently bought a NOS: PLATE, Engine Front Mounting, Group 0.004, part number 3709945 (1954/55st). It differs from the 37-53 plate by having a 2 3/8" cam hole vs the earlier (1937-1943) 2 1/4" cam hole.
It has a nut (no washer) on each of the two bolts. The bolts seem to be as Denny described above: with respect to the heads and how they are attached/pinched to the plate. The slot on each side-face of the nut comes down about 1/3 of the height of the nut. There is a 1/16" slot and it is over to the right of each of the six side "faces".
Let me know if you want photos posted.
