BA screw size for Shackleton yoke?

I have finally got round to removing a bit of the plastic coating on my Shack yoke to expose the tapped holes to attach the two large push buttons. Not having any engineering background I wondered if somebody could tell me what size BA screw I need to order to secure them with? Also I do not suppose anybody has a set of brake levers sitting around not needed that they would be willing to part with?

Thank you,

JDH