Per Jerry's post above about dirt getting into the system, I have seen where folks back in the day added a nipple to the valve cover and another one to the top of the air filter and connected them with a length of rubber hose to allow filtered air into the engine. The packing in the oil breather caps doesn't offer that type of protection and open slits in the valve cover certainly don't.
I have seen old engines with this type of installation that have had the valve cover slits brazed or welded shut. I have seen it done with homemade nipples and ones with 90 degree bends and flat mounting flanges that took two screws and a gasket that look like they must have been sold as some kind of a kit because they are all the same and look as though they were made for that very purpose.
I seem to recall that Jerry has posted in the past that California passed early emissions laws that required installation of a PCV and a filtered breather system. When I see the retrofits mentioned above combined with early PCV plumbing that was obviously installed back in the day, I figure that it was probably a CA vehicle. But I sometimes find the venting retrofits on engines that still have road draft tubes, so some people were obviously simply trying to let the engines breath with filtered air regardless of emissions laws for the reasons described by Jerry.
Matt
