[ajmac]

Good luck investigating.
Attached is a little single line sketch of the various Flathead V8 oil system layouts. Yours is the third one down, at least on the face of it!

On a well designed modern oil sytem you would have a PSV (Pressure Safety Valve) around the pump to protect from dead heading, this would be the highest pressure setting in the system and shouldn't lift in normal operations. After the pump you would have the main PCV (Pressure Control Valve) for the system and after that the TCV (Temperature Control Valve) directing some oil to the cooler and mixing it with hot oil to maintain a set temperature before sending it on to the oil filter and the things you want to lubricate! You would monitor the line pressure after the filter just before the things you want to lubricate.
In an automotive application it would be quite acceptable to combine the PSV and PCV into one PCV as part of the pump.

PS. The rear main is fed directly from the pump discharge before it ever gets to the top of the bell housing, thats what makes fitting a full flow oil filter almost impossible.