What is the sensor/fitting on the top rear of block, in the rear oil gallery wall ( if in car next to the firewall ) it has one connector lug on it and is silver colored

If you are referring to the small threaded hole on the top edge of the block just behind the intake manifold, that is where the oil pressure sender or mechanical oil pressure gauge line connects.

We're assuming you're talking about a small block Chevy................

Thanks , thats the one in question. Yes it a 1971 ,350 SB