I recently upgraded the "buttons" on my Nova Cole jaws with #3 Black lab plugs, m6x1 shoulder bolts w/ 8mm x 25 mm shoulders and 5/16" washers.
I had to drill a recess in the plugs for the size of the washer so the shoulder would bottom out on the jaw as well as a 5/16 through hole. This worked out great as I now have a taller button and more secure hold on my bowl and I don'y have to worry about running the stock screw through the button. The length of the plug and bolt work out great for a secure fit.
I Should have done this log ago.