I think you were very lucky not to have that piece of metal ingested into the engine. Is it a piece that was part of the air filter canister assembly?
Use carburetor cleaner to clean the carburetors as best that you can.
You can try smoothing the piston with 0000 steel wool of maybe 1500 wet/dry sandpaper, but don't mess with the top lip interface between the piston and the dashpot dome. The piston interface is supposed to be machined to match to the dashpot.
https://youtu.be/IfU47Oqq9wA
It almost looks like the black are oil deposits. Does your car have an crankcase ventilation system where there is a hose running from the block to the carburetors.