Yup, a mandrel of some kind left that behind, squish cutting, holder for base cutting or old piston for squish sanding.
Looks like it may have been a tight fit and had to be wiggled to get it out and had some chips stuck in there or as already said a cutter with minimal clearance.
Odd that it's only part way down.
Does appear that more than one pass was required as the patterns vary so much.
May or may not be an issue depending on how deep they are.
Being able to feel it isn't encouraging.
Piston doesn't seem to have suffered for it.
Seems like the builder should have been contacted first and this posted publicly only if the response was unsatisfactory or sketchy.
That and out him or don't bait us with this stuff at all IMHO...
Hope you keep us informed of the cause when this gets straightened out at least.
