Its hard to see, but if you look at the photo I posted of the parts, look at the stem where the threads are for the cap and you will see a ring of threads that look darker then the other threads on the stem. Its actually a little piece of leather, a ring of leather (with threads from the cap in the leather) that acts like a seal for the cap. The later metal replacement stems don't have that little ring of leather.
As for the little disc, its not actually a plug. Grease needs to be able to get around it so it is actually small then the id of the stem port. Its entirely possible that the disc was a piece of bakelite with a piece of fiber bonded to the bakelite. I have never seen a nos disc so by the time I get a distributor apart, its either worn to the nub or missing entirely. Like I said, I used a piece of delrin rod and had a friend chuck it up on a lathe had machine a few down to the same diameter of a worn out disc. The finished piece is on the right in the photo. Also, later NOS 2403 distributors came from Delco Remy with plastic stems and cups, and those same plastic cups and stems were sold over the counter. The plastic parts also did not have the little ring of leather.
Below is an nos 2403 replacement distributor with the plastic stem and cap. It’s discolored but you can see no leather ring. The replacement plastic caps and stems are also below, same deal, no leather seal.
