I spent way too much time trying to sort this out on my Grand Banks and again on my Nordhavn. The problem is that some boats (all probably get fittings from the same source in Asia) use standard 1-1/2" SAE pipe diameter, but with 14 TPI (threads per inch). The adapters that everyone sells are 1-1/2" 11.5 TPI which is the NPT (National Pipe Thread) standard. They don't fit. I have never found, anywhere, an off the shelf adapter for 1-1/2"-14.
I ended making my own with the aid of a machine shop. I had them cut 14 TPI into the end of a length of 1-1/2" Sched 80 PVC pipe. I then glued that stub via a coupler to a banjo fitting to mate with standard pump out hoses. I used PVC so I could position the coupler to create a ridge to retain an o-ring so the fitting would seal to the deck plate. That part worked well, but the threads did not cut well in the PVC. It wanted to chatter in the lathe, so the threads are rough and really don't mate well with the deck fittings. If I were doing it again I would probably use steel or AL. The problem is then figuring out how to get the retaining ridge for an o-ring so it seals. I originally priced having one machined and it was through the roof, which drove me to the PVC approach. Machining threads into a PVC stub is cheap, and I actually had a bunch made. From there you can glue it up to pretty much anything you want.
