I'm looking at your pictures and wondering....the bungs, the large ones...what NPT comes of that "buttress adapter" (I looked at the link but didn't see that spec)? Do you have any flow problems out of that?
The top bungs, those look much bigger than the about 1" bung on the barrels I've seen. What diameter are they? Is that grey water inlet through those?
On my drums there were two different 2 inch Bung holes on each drum, the bottom one in the picture is a course thread, the top is a fine thread, the adapters that I bought adapted them to 2 inch standard NPT thread, the same thread you would find on a 2 inch PVC pipe in any Big Box store.
The drum on the left is for Grey water, the pipe coming into the top bung from the left is Grey water, there is also a grey water inlet/vent going into the top left side using a 2 inch Uniseal.
The elbows, Tees and valves connecting the two top Bungs are for flushing purposes, the valves are normally closed, when a hose is attached to the center hose fitting and turned on and you open either side valve it dumps water into that drum to flush them out. It could also serve to dump grey or black water from one to the other if one became too full, I have not used that feature yet.
I have never had any flow problems, with the Stinky Slinky attached when I pull ether of the drain valves the tanks empty in moments, due to the location of the Bung holes there is always a small amount left in the bottom of each tank, but it has never been an issue, neither of the tanks smells particularly nasty, I flush them on a regular basis until the water runs clear and we always use Happy Campers tank treatment on the black tank.
On the fresh water tank the Bung caps that came on the drum had punch outs in the center that were ¾ inch NPT standard threads.
Peter