Ford Truck Enthusiasts Forums - Enginer number question on 51 f3 v8

I'm not sure if Ford flat V8's ever had the chassis serial number stamped on them. Up until the late 40's, the chassis number was stamped on the transmission housing bell flange, as shown here. That practice ended by the time our trucks were built. The only serial number stamping is on the frame rail.
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The "Engine Number" designation comes from the common practice at the time of using an engine number to register vehicles. That quickly became an unreliable method as engines were routinely swapped out with remanufactured units or replaced when broken, and the number was subsequently lost unless restamped. Restamping was a common process and stamp sets were sold to dealers and rebuilders by the vehicle manufacturers. My how times have changed.

From Van Pelt's website...Early Ford Serial Numbers For Cars and Trucks Manufactured for the USA Market from 1932 to 1953

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.

The number (including the prefixes shown) was always preceded by (and followed by) a stamped "star" similar to an asterisk. It is believed that this was done to prevent someone from fraudulently adding a digit to the beginning or ending of a serial number in order to alter its identification. From the Ford Service Bulletins there is a bulletin dated April 15, 1938 that refers to "Engine Numbers". The subject covers the use of the correct stamps when reconditioned cylinder assemblies are stamped by dealers. The bulletin goes on to say that the dealers should use the K.R. Wilson number A404B stamp set. The 1940 K.R. Wilson tool catalog shows the A404B stamp set of 11 stamps (numbers 0 thru 9 plus the "star"). The same catalog shows a new number A404 stamp set of three (just the 6 stamp, the 9 stamp, and the I stamp). The K.R. Wilson catalog states that "The design of engine numbers has been changed to lessen the possibility of outside individuals attempting to change engine numbers. The figures I-6-9 have been changed. Dealers should immediately provide themselves with the new figures - One-Six-Nine. See Ford Service Bulletin, Subject No. 6000 (Engine) Page 22."

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The diagram above is from the 1940 K.R. Wilson Catalog of Service Tools & Garage Equipment, and was included with the notes about the change in stamps. The primary differences were the numbers 6 and 9, and the number 1 becoming more like a capital "i" in appearance.