I'm sure there are a lot of originals out there, looking like they have a clump of dirt on the underside of the cap, but that can't be good for the fuel system when bits fall off into the tank. It's actually not even rust, it's a type of bacteria that turns into a rust-like substance. I use a fuel additive for moisture removal (Power Service) and it still happens, mainly because I spend a lot of time in the Midwest and the South where it's so humid.
I considered spraying or panting the cap with some kind of sealant, like acrylic or polyethylene, but I don't want that falling off into the fuel, either.
But the last time I changed mine instead of using another fuel filler cap I went ahead and used an
oil filler cap. I had a spare oil filler cap, because I once added half a quart of oil when fueling and dropped the cap. And I couldn't find it. It never came out and hit the ground. It fell down the left side (or right side? Passenger side) of the engine and it was dark and I couldn't find it and people (all driving really big trucks, and one Pilot manager) wanted me to get out of the way. So, I dropped the hood and started up the van and moved it 100 feet. In that short span I lost 5 quarts of oil. It was a sight to behold. The next day I sprayed the engine and most of the compartment down with brake cleaner and a couple of weeks later replaced the matting under the hood. If you ever drop your oil filler cap and a big truck runs over it or you somehow lose it, use the fuel filler cap as an emergency replacement.
I finally found it, wedged between the engine and the exhaust manifold.
I replaced it with the oil filler cap a few months ago. The only downside is the oil cap doesn't have that nifty lanyard attached to it. The upside is, no more rusty crap and the the oil cap it $4 cheaper than the fuel cap.
Fuel Filler Cap
Oil Filler Cap
The little tabs aren't really exposed to the filler hose, thus no rust to be found on those, and the bacteria that causes the rust doesn't react to the plastic.
Here's mine.