oiling questions for 475cu in a L-800
 

I've got a L-800 boom truck and I'm trying to find the oil sending unit, I want to install a manual pressure gauge to see what the oil pressure really is, but I don't seem to be able to find the sending unit. I found one sender at the rear of the block but when I unhooked it the gauge still worked (showed pressure). Anyone have an idea where the oil sending unit might be located on the ford 475 cu inch engine? It doesn't appear to be anywhere on the top, front, or rear of the engine.

Thanks in advance for your ideas.