Sounds like a scratch in the piston. Over time, and after ingesting bits of foreign
matter...an engine will exhibit these. If you fly in a dirty, gritty location...expect to see
more of these. A bru-line air filter is a cheap means of limiting the amount of garbage your
motor is able to breathe in.

As long as you say the compression is good, and the motor is running fine, I wouldn't
lose any sleep over it.

Since there really isn't much in an LA motor to corrode...you might consider merely
keeping the motor tidy in your daily post-flight clean up, and minimize your disassembly of it.

Oil it up nice and it's good to go.

'Race