Peter, My system is not stock at all. I put it in to replace the overboard discharge that the factory put in in '75. My deck fitting is stainless, and has just the thread for the pumpout. I have never seen the system you describe. Marinette must have changed it before '81.
1975 32' Flybridge Sedan, twin Perkins 6-354 diesels, 1:1.53 velvetdrives, 16 X 19 props. Merritt Island, Florida