Well, I'm glad I'm not the only one interested in paint daubs and inspector stamps!! I have four 1968 San Jose Mustangs (one GT, two GT/CS's and one coupe) and I have found four inspector stamps on these cars under the hood. Three stamps seem to be paint inspections, identified by a "P", and one stamp is an "OK" stamp.
Here is what I've found: All stamps appear to be yellow in color (I used Testor's yellow model paint to match the original). The height of all stamps I have found are 1/2" and the length of the stamps are 15/16".
One stamp was found on the underside of the hood on the passenger side 6 1/2" from the front leading edge of the hood and 1 1/2" from the right side of the hood edge. (This stamp says "P-24").
Another stamp was found on the left front fender, 2 1/2" behind the rubber hood bumper on my GT/CS and 2 1/2" behind the first fender bolt on my GT. The stamping mark reads from the engine side, or is "upside down" when reading the mark as one would look into the engine compartment. (This stamp says "P-30"). Both the GT and the GT/CS are Highland Green cars, but I don't think this is relevant to this stamp.
Another stamp was found 38" to the rear of the first fender bolt on the passenger side of the right fender. This stamping also "reads" from the engine side, like the left fender. (This stamp also says "P-30).
The final stamp I've found is located on the hood latching bracket on the left vertical side of the bracket. This stamping is 1" from the left side of the bracket, just below the two left bolts attaching the bracket to the radiator support. (This stamp says "OK-56").
When I found these inspector stamps, I went to a local stamp and engraving business and had stamps made that would replicate the paint stampings. There may be more inspector stampings on 1968 Mustangs, but I have not been able to locate any other paint stampings on my cars. I hope this helps, and good luck with your restoration.
Russ