Quoting from the Ford Automotive Assembly Division Data Processing Manual for 1967, "Special Order Number Note: When these columns (the DSO) are punched, there must be a punch in column 80. When the special equipment specified on a DSO or FSO is one of the basic items, i.e. series, body and/or engine, the item will be punched..."

What this means is since the P or Q engine was not available for the Mustang under ordinary conditions, a DSO had to be generated. This DSO was about 11 pages long and contained notes about what items to delete (in this case the 390 engine amongst a host of other items like the front fenders and hood) and what items to add (amongst them the 428PI engine). Since the column for the engine (column 5 of the IBM punch card) must be punched with something, they made a decision to put the nearest appropriate engine code (an S) in this place. It is a place holder required by the programming system. It means nothing other than that. Whenever there is a DSO, it superseded all other punches for production.

Once the order arrived at the assembly plant (in this case San Jose), the assembly plant manager made a decision to insert the "Q" in the serial number to be stamped into the fender aprons. Since this operation was performed manually, it would not be prohibited by the system. But note that the actual motors installed were not even Q motors, but P motors.

I hope this gives a little insight into why things were the way they were.

Kevin Marti
www.martiauto.com