Pre 38 (Double check your frame first)
**Old style refers to years before 1938
**options A and B are for what letter you want included with the kit. Letter A or Letter B
Below is the difference between the new and old number stamping. If you're interested in the new numbers please let me know. Info from (http://www.vanpeltsales.com/FH_web/flathead_serialnumbers.htm)
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 (click here for a picture example). 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.
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."
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.
On 1949 and 1950 cars the number was stamped on the dash panel (firewall) under the hood. In 1951 the number was stamped in a plate which was mounted on the dash panel (firewall) under the hood. For early 1952 models the number was on a plate mounted on the right front door pillar post. The late '52 through '55 models (except '55 T-Bird) had the number on a plate on the left front door pillar post.