not mine, but it looks like the person marked a circle by tracing the half dollar crimp on each end soon after adding the letter c in the middle, so odds are, for every one marked like that equals at least 1/2 a roll(10 coins) of the person's dumps, i hope that makes sense...
i might mark 20 coins per box, but if you get 20 of my marks in a box, chances are you got a full direct dump.
hh
buff