This question has come up at a local club meeting a couple times, so it is not uncommon. Here is my take on it. It is a bit redundant but includes an analogy.
When i first viewed the internal workings of the Model A oil pump, I realized that it is essentially a pair of gears being driven off the engine that are moving the oil. The Model A pump is like the pump in the rear case of a general aviation piston aircraft engine. My own aircraft will pump 100+ on initial startup on a cold morning. The gears in that pump are smaller than the Model A gears, but may be turning at a faster speed.
Liken the low pressure seen on the Model A gauge to a garden hose, where the spray nozzle has been removed from the output end and the faucet turned wide open. The only restriction would be the friction drag of the fluid / water flowing through the hose. When the spray nozzle is reattached and set for a jet stream, the line pressure rises, shooting the small diameter stream several feet under pressure.
If the Model A pump were pumping oil through a labyrinth of passages to main and rod bearings, overhead valves etc. with only .0015 thousandths openings at the far end, it would show normal engine readings of ?? pounds.
As a young man, I had a friend who was a Model A engine builder that specialized in winning engines for Model A hill climb events. He was pressurizing the mains and adding hidden nitrous oxide bottles to the stock Model A engines! There are some old time racers on this forum and I seem to remember some quoting readings ... 25 50 and up.
