Coolant filter?

[HO455]

They've been around since the 30's or maybe earlier. Many poor folk back in the day couldn't afford anti-freeze. Come winter they would drain the water and not drive the vehicle until spring or if they needed to drive somewhere the would start the engine and then add water. They would keep the engine running till they were home again, where the water was drained back out. Putting a screen in to keep the rust and dirt out of the radiator was cheap insurance for the radiator. No one back then put distilled water in the radiator. Water from the well, or horse trough, or the ditch next to the road was good enough.

They're still pretty popular with the pre-war restoration crowd. Old engines that don't get run a lot and when an engine sits over time the anti- freeze can start to separate from the water and you get rust in the lower parts of the block. Then that rust ends up in the radiator. The screen captures the biggest pieces of rust.