cummins 855 block, what's this fitting for?

i have a commercial engine i took out of an M818 and it doesn't have a fitting in this location but it does have a threaded plug in that spot .. It's right below the accessory drive and it dumps into the top of the secondary (lower) accessory drive

Is the fitting seen used as a place to dump oil back into the oilpan after going through an external accessory? thanks in advance .. (waiting for will wagner to pipe in anytime, too !)

well, no takers or guesser, but thanks anyway.. I finally have it squared away on my own by taking off hoses from the other truck and matching up the end fittings to this one

It's for the oil return line coming from a big Luberfiner external engine oil filter, which is something that no other engine has around here .. The thing is almost the size of an air cleaner !