The top of the oil separator has three threaded holes, two should be -10 size and one larger at -12 size.
The two -10 size holes are for the inlet to the separator from the engine, usually this is from the original place in the cylinder block below the inlet manifold, the second is taken from the base of the rear of the cylinder block on the exhaust side.
The larger threaded hole in the separator is the outlet, and although this is threaded to suit a -12 fitting we usually use a -12 to -10 reducer in here on road car engines, using a -10 hose from here to the oil catch tank

The oil return to the sump AN6!!