Can anyone help explain what could cause this kind of damage? I'm guessing overheat?

This is out of the #5 cylinder which had zero compression when I bought the car so I don't have any history. The exhaust manifold heat riser was stuck closed which may have contributed to a possible overheat issue.

 

Looks like detonation to me.

I had similar damage to one piston in my 1970 Cutlass Supreme (10.25:1 compression 350) in 1982 after getting some cheap low octane gas then running the engine up to 5000 RPM.

 

What did the crank bearings look like?