There is always the possibility that it has been swapped in. According to all the resources I check, Chrysler's transmission part numbers began with the number 3 up until 1976. A late 70s Diplomat would have a part number beginning with the number 4.
IF the information I sourced is correct, you should have a non lockup 904. The Diplomat would have came factory with lockup. I'm assuming it is still bolted to the engine? If not, have a look at the input shaft. Lockup will have a machined nub, non lockup will be plain splines.
