The seller of the 1985 CHP Mustang in New Orleans sent me some pictures of the documentation he has with the car. In it, there are two pages described as the Special Equipment Parts List, from Ford:
Note that the total number of units shown for DSO order #72-0406 was 154 units. Everyone has been operating on the basis that the CHP bought 155 SSP's that year. Either the number was in fact 154, or the CHP purchased one car separately from the above order of 154 cars (perhaps as a test vehicle for the new model year). Regardless, it's good stuff!
Also part of his documentation was an extended warranty letter that shows the VIN for CHP #E408327 -
If that car ever turns up then the owner with that VIN will know the car's CHP Unit Number.