I could be missing something, Bible, but I don't recognize anything special. Remember using plumbing parts like this for air lines. You can see the marks from a pipe wrench being used. I don't remember "dedicated" plugs ... often saw manufacturer's marks on the ends, was all.
I tossed used parts like this in coffee cans, just in case I needed something in a hurry. Usually took more time to clean them and check the threads, but they would work, and save a trip to the store. You've got a coupling and an end plug ... as is, they give you an end cap, also. Kind of depends if you're more of a fix-it kind of guy and might need something in a pinch. The parts are cheap, so it usually felt better to just pick up a new one at the hardware store.
I assume that you're asking what the metal is that the iron oil plug is threaded into?
It almost looks like zinc, but I'm now thinking it's galvanized iron.
Dave
"Galvanizing is one of the most widely used to methods for protecting metal from corrosion. It involves applying a thin coating of zinc to a thicker base metal, helping to shield it from the surrounding environment. The next time you are in your car, take a look at the street signs and lamp posts you pass. A large number of them will have a mute, silver color on them. That “silver” is actually the coating of zinc. Quite simply, galvanizing a metal gives it anti-corrosion properties. Without the protective zinc coating, the metal would remain exposed to the elements and potentially oxidize and corrode much faster. Galvanized Steel is a cost-effective alternative to using materials such as austenitic stainless steel or aluminum in order to prevent corrosion."