What on Earth caused this?!?

I was cleaning up an .049 from a tether car that I got a few months ago, and when I pulled the cylinder off the motor and removed the head, I found this:





You can see between the blue lines the large trough that goes the entire length of the cylinder.  This is between the exhaust slits on either side.  The red arrows point to two more, but smaller areas of the cylinder where it's wallowed out.  One of the two you can just see through to the other end of the cylinder if you look just right.  And, as you can see from the picture, there's a small gap around the top of the cylinder from the 6 o'clock position to the 2 o'clock position, but it's just at the lip of the cylinder.

I couldn't figure it out.  The piston itself doesn't have any scarring or damage marks on it, and it's perfectly round.  After thinking on it, I guessed it might be out of round somehow, maybe from an overtightened heat sink.  But using my calipers on the outside of the barrel and fins showed the external dimensions to also be perfectly round.

Has anyone seen anything like this before?

The Stumped Mark