I pulled the stepper control board out of the blue box to clean, inspect, and identify suitable PCB locations to patch in the Mega2560. There was a small amount of caked-on sawdust, and the heat sink compound was dried up.
The back side of the stepper board is rather unremarkable. Note the snubber diodes on the left side, directly underneath the motor connectors. Good PCB layout choice.
I noticed that the corner of the stepper board was "trimmed" to fit the inside corner of the box.
Also, the heat sink was "modified" to clear the plastic foot screwed into the underside.
Note, the driver chips are mounted to the heat sink with a triangle head screw, so you'll need a driver tip that looks like the following. I have a driver bit mega-kit that included one Alternately, you could remove with needle nose vise grips, and use Philips head screws.