The 1937 American LaFrance V-12 I have has a Pur-o-lator reusable oil filter on it. It is essentially a stack of round brass shim stock in alternating layers in a pattern to produce a cylinder with some gaps a few thousandths tall and wide all over it. It filters out things like rocks, sticks and the small birds that carried in the rocks and sticks. I'm not sure its terribly effective at filtering out particles that could be damaging...On the bright side, once the oil leaves the filter on that engine it does pass through an oil to water cooler, so at least you have cool oil with small chunks circulating back into the engine.
From the outside it looks like a canister filter attached to the top side of the oil cooler. The brass element fits over a hollow tube and works similar the bypass filters commonly found on our GM engines, but with a larger hole to allow full flow.
