The fourth character in casting/part number prefix is for the Design Engineering Office that created the part.
| A | Light Truck Engineering Division |
| B | Body and Electrical Product Division |
| C | Chassis |
| D | Overseas Product Engineering |
| E | Engine |
| F | General Parts/Electrical and Electronics Division |
| H | Climate Control (1972-up) |
| J | Autolite/Ford Parts and Service Division |
| L | Industrial Engine Parts and Service |
| M | Performance/Special Vehicle Operations (Incl. Holman Moody) |
| N | Tractor Parts |
| P | Auto transmission |
| R | Manual transmission |
| S | Light and Heavy Truck Special Order Parts |
| T | Heavy Truck Engineering |
| U | Special Vehicle Operations |
| W | Axle and Driveshaft |
| X | Emissions, Economy, Special Vehicle Engineering (Muscle Car Parts) |
| Y | Lincoln/Mercury Service Parts |
| Z | Ford Service Parts |