Might be pressed, a very fine bend in it. The lighter area you describe is where the bend is so possibly was under a screw head. Another pic showing the bend at the top of the fragment.
Engine was rebuilt in 2012... I have only put about 200 miles on it since importing it last year, I estimate only about 400 miles since rebuild.
As the engine appears to be running ok I was thinking it may have been shrapnel left over from the build or previous life.
I should have said the hole is on right/passenger side opposite to starter.
