I got an anwser from the guy i bought the pump from and he said it did not matter which was it was wired. I wired it up and plumbed in a pressure guage and isolation valve. I ended up with the #2639 model and seems to work good. The pump will create up to 40 psi before the relief valve vents. Seems to put out at least double the air of my Thomas 5120S linear pump. I will be hooking up this weekend........will let you know how it goes.