It was a button head type machine screw.....Allen drive.
And a Weber lock nut that came with the daisy wheel kit.
A Gripco type lock nut would work too.
A lock washer is not a good choice for this application because it's a connection that must move.
The lock washer solution would eventually loosen up.
The Gripco is a deformed lock nut and does not have a nylon insert.
You can see the press marks on the side of the nut.
Break in the Gripco on a regular bolt first, so you will have an easier go when switching to the Allen wrench.