Sorry, yes you are right. It's an
oil level sensor, I just looked it up. I'd assumed it was a sump heater connection for cold climates since the Aussie motors didn't seem to have it. My Holden sourced loom didn't have a connection for it either.
I've connected the steam pipe to the water pump but I made a filler neck and pressure cap fitting and welded it to the pump. That let me delete the header tank. Previously I had the steam pipe connected to the header tank. I reckon it needs to go to or near the highest point in the system to purge any trapped air out of the system.
How it's set up now.
Cheers