there's no oil sending unit on the manifold.
That's because there is no oil in the manifold

What make motor we working on here Marty? The oil sending unit looks like a small can with a wire terminal at the top of it (no matter what flavor noise maker) and will be installed on the block, I believe an Olds will have it in the fron of the engine near the water pump cover, a Big Ford will have it near the oil filter, and a Big and Small chevy will have one near the distributor.
The easieast way to check for "actual" oil pressure is go go buy an "el cheapo" mechanical gauge and install it in the 1/8" pipe hole in the oil galley, just don't leave it there permanent like...Good luck and hope it turns out to be something minor...
GT