As far as I know all SPF car numbers do start with SP, as you mentioned. Are you sure the VIN the car is registered under isn't what your looking at? Almost sounds like someone registered the VIN for the car, playing with the CSX letters of the original Cobras. I got to pick the VIN for my SPF when I brought it into Calif. I chose to make it the build number SP01038.
Find out the number stamped to the frame cross bar (in front of the engine).
Also, look at the trunk hinges where they bolt to the trunk lid. The SPF build number is on both attaching points.
