What is inside an oil filler/breather cap?

All the ones I have seen have a course steel wool like material in them.

Mine 67 dart 273 does not seem to have anything in it.

I don't like that cause the pickup point is before the air filter in the snorkel so it sucks unfiltered air into the engine.

I have thought about filling it but was hesitant for the reasons listed above.

Maybe a plastic fiber mesh oiled down would be the ticket