One way that metal filings can get into a clean motor is from the new
oil cooler lines. If the assembler didn't fully flush them out after cutting the braided stainless, some rubber debris and stainless debris will be stuck to the inside of the hose. Flow some
oil through it and it will get flushed into the motor and cooler. Most people just blow the lines out with air after cutting them to length, I always would flush with: air, solvent, then air again to get everything possible out. Or the engine builder could have let the motor sitting around without a distributor (or intake, etc) in it allowing stuff to find its way in. Either way, it's in there until you get it out. A set of gaskets is cheap compared to a new rebuild.