I know what it is...it's something Jabsco tried in the late 70's that turned out to be something of a disaster...it didn't last long. I first ran across one in 2011...had no idea what it was, so sent a photo to people who'd been in the marine industry for a few decades and got this reply from Jock Allpress: "I worked for Jabsco in those years and do remember the fitting you are showing. It appears to be a plastic fitting which by the time would be of little use. One of the reason the impeller discharge pumps have a 1-1/2" threaded end is because the original thought was that you could screw that in to discharge though the deck plate, but it was never adopted because on the mess issue. There were a number of blow up disasters trying to pump through the deck discharge plate."
The idea was a combination pumpout and macerator discharge deck fitting...the larger 1.25" opening was the pumpout fitting, the smaller 1" opening was the macerator discharge fitting. The pumpout part worked ok...but spewing the contents of a holding tank out a deck fitting didn't. If the operator wasn't careful to make sure the fitting was always on the lee (downwind) side of the boat, or if the wind--even a slight breeze--shifted, the result was a mess that really doesn't need more description. Jabsco tried putting in it the side of the hull, but that didn't work much better so Jabsco scrapped it as a bad idea. Yours is the 3rd or 4th one I've come across, all on boats about the same vintage as yours--one of which was a Grand Banks who was at the Puget Sound GB Owners rendezvous at Roche Harbor several years ago...his was also in the side of the boat as yours appears to be--none with owners who knew what it was but oddly enough, left in place...as a conversation starter I suspect. I have no idea what Jabsco called it...my name for it is the Jabsco Poop Shooter and have often thought it might add a new dimension to water cannon fights.
So the bottom line is, you'll have to install a normal deck pumpout fitting and a thru-hull (below waterline please!) for any overboard tank discharge.
And btw...welcome aboard the Trawler forum!
--Peggie