Ok. I think I've finally got this figured out. Since your original post, you'd been saying "on the block". But in your last post you say that you put the stock sending unit back in, & asked what was that to it's left. This leads me to believe you were never working down on the engine block, but rather on the cylinder head. The larger fitting is the engine coolant temp sensor (red & green wires), the smaller one to it's right is the e.c.t. sending unit.
I think we're done here (fingers crossed
)
-W-