Maybe this will help. "From 1932 through 1948 vehicles, Ford Motor Company stamped the serial number into the top of the driver's side frame rail in three locations. This same number was also stamped into the top of the transmission-to-engine housing portion of the transmission. This location was visible when the floorboard and transmission cover plate were removed. Of the frame-stamped serial numbers, only the forward-most stamping was visible when looking down into the engine compartment (located between the front crossmember and the dash (firewall). The two other frame locations are visible only when the body is removed from the frame. The serial numbers were metal stamped with numbers/letters approximately 1/4" in height."

Sources: [url]http://www.vanpeltsales.com/FH_web/flathead_serialnumbers.htm[/url]

Not too aware of Mercury Serial numbers, but is it possible that the serial number is the same as the engine number? Not sure if I'm right, but just an idea.