The serial number is stamped into a small plate on the firewall near the voltage regulator. Mine is stamped in small 12 pt type. It was also supposed to be stamped on the bellhousing for the early tractors, but I couldn't see it on mine. It is a 1953 model, s/n 1,251,624. Later they would have stamped it on the pad on the engine block behind the distributer.
The serial numbers are listed on the left side of this page. Fordson used the same sequence from 1917 to 31 Oct 1961. (Except Dextas) Then they started a new numbering system.