It's burned RTV, Rick. Recall that the heat from the center mounted rotors fried the seals back in 2011. I didn't RTV that area when I replaced the seals, so it must have come from Mike Bontoft - CWI, who built the diffy. Curious why he would seal it up if it is supposed to be open. No flakes.
If I understand Doug correctly, the hole drains the outside rim of the axle seal. It has two purposes, one is to keep
oil off of the brake rotor when the seal leaks, and the second is to act as a telltale that you have a bad seal. No brake rotor there anymore, so this is really just a good housekeeping exercise. I hate drips
Yeah, it's always the right side seal that goes, never the left. We need to run more races clockwise ...
Sam