But I don’t fully trust that piece of info because of this post (about the 4011 that has a different way to mount ears):
http://forum.mikrotik.com/t/4011igs-rack-mount-screws/145664/1
So we have at least two measures, M3x6 or M4x8.
Very likely the ones used are the M4x8, but it is easy to check, the threaded hole should be either 2.46 mm or 3.24 mm, that in the measures of our friends across the pond should be roughly 0.097" or 0.128".