Why does the oil cooler element need to be indexed? M923

[1 Patriot-of-many]

I'm in the process of changing my oil cooler on my M923. It says to index the oil cooler with the mark on the housing(the marks on my housing seem to be just chisel marks, nothing from the manufacturing process. I cannot see what the reason would be, there is no matching tubes on the engine or the oil cooler housing they are both wide open. There was no index tab on the cooler element I just removed. Just wondering what the purpose would be. Anyone?