I've never seen a Legnano with the serial no. on the downtube. It's probably a personal ID number. Check at the top of the seat tube for a serial no. Also check for a small threaded fitting brazed to the back of the seat tube at the top. If it's there your Legnano is 1961 or newer. If no serial no.stamped into the seat tube check underneath the bottom bracket for a serial no. Around 1966 the serial no. appears to have disappeared from the seat tube. Very nice find, consider yourself extremely fortunate as that Legnano looks great.