Ok let me rephrase that; mine were special.
They have a widened base as part of the nut, such that the nut doesnt drop into a deep socket, but rather lands on the widened base. The base of the nut when viewed from the hole end is round like a washer, while the other end is a hex..Furthermore the nut is a black. Not painted or plated. I assume its a hardening treatment. And finally mine have been dimpled on the side to create a mild self-locking effect. I cant say whether theyre original or not.
So, sorry if I am in error.
I would put a high grade nut and hardened washer on there. I think the torque on that thing is maybe 130lbs.