Yeah, it's that little bit of plastic. I know the bearings are already shielded, but anything that helps keep a little bit of extra moisture out is ok in my book, especially since the pedals are already about 20 years old. I'm probably just going to try to fabricate something new. Maybe I can even adapt some manner of light rubber gasket to suit my needs.
I love my Superbe Pro components. I'm already looking around for another set for my next bike.
Side note: The hubs I have don't take cartridge bearings, but they look identical to the (presumably later) models that do. They're just super nice loose ball cup-and-cone hubs, with full "Superbe Pro" branding. I can't find any info on them online, though. The whole bike/grouppo came as a hand-me-down, and the previous owners who had any idea about the specific history of the build are nowhere to be found.