Would guess that its an early 70 CJ trans Most CJ's Mustangs are not listed in the registry. Higher percentage of that being correct than being one of the more limited 70 Boss 9 trans
Could always (takes time though) ask for a Marti report on the car with a completed number for a Boss 9 with that sequential number and see if they find such a car.
Guess you've already checked with the Boss 9 registry though allot of those cars aren't in the registry also