Lets CSI this shit - piston caught something on the top edge of the transfer and it gouged down the side of the piston. twice. all we know for sure is that there was (or maybe still is) something in three that caught and gouged. is there any shit still wedged in there to indicate what it came from?
size and shape matches pretty closely with a needle, how's the crank condition. I had a bigend bearing explode on a ZA once. nightmare status. Also circlips - are they all still there?
circlips and bearings are likeliest to catch on the transfers like that. stll anything getting in the cylinder is going to have a chance to catch on the transfers. Weld shit can screw you, it's possible to suck shit back up from the exhaust but still... not very likely.
Anthing missing from your carb>? I dropped out a throttle slide retaining pin from a clone bing once. never used a clone bing since...