I've owned/parted out a couple of those police diplomats. IF they came with the oil cooler, one of the fuel pump bolts are hollow and an oil return line is fed through there. I don't have a picture of one, those cars are some years in my past.
Was there an adapter between the oil filter and the block? Where did the oil start from? Assuming the cooler was in front of the radiator.
I had an '88 Diplomat cop car and it had the auxiliary trans cooler and PS cooler, but don't remember it having an oil cooler.
Still have the huge alternator bracket and 110 amp alternator, PS cooler and (maybe) trans cooler. Time to trash some of that stuff. Except the PS cooler, that's small and too cool to get rid of.