I have 5 fitting on our 78'.
2 lower glassed in are the cockpit scuppers.
Starboard 1.5" transom fitting goes to auto bilge pump
Centerline 1.5" exhaust from motor.
Port 1.5" manual bilge pump. The hose runs from the pump located inside of the forward bulkhead of the sail locker roughly near engine control panel (you have to insert handle and pump it by reaching into the locker).
On ours the hose runs from bilge to pump then from pump aft to fitting via under the quarter berth.
A previous owner had put a y valve on this and was using the pump to drain the poo holding tank.
My guess is something similar to this is what happened in the past to your boat. That white hose is a waste type hose.
I put in some new bilge pump hose and it works as designed, I do want to find a better location for the pump. I like were yours is, hmmm.
Our exhaust hose runs thru that hole as we have a flapper in the sail locker.