Hmmm - that was a couple of years ago, and I've since sold the boat, but I found a picture of it. It would be a 1/2" nylon T, which will work with the factory water hose if you slightly heat the ends before pushing them on the fitting. The picture shows a strainer; I later removed that and plumbed the head intake directly to the T because the strainers are not rated for pressure, and I had two of them fracture and flood the cabin with fresh water! You don't really need a separate strainer anyway, since there is one on the fresh water pump intake.
P.S. Probably came from WM - one of their more reasonably priced items. The hose connecting the head intake to the fitting is 1/2" ID reinforced fresh water hose, available anywhere.