I found a few spare minutes to indulge the OCD in all of us.
Mystery solved. I have a single disassembled 240 rack housing. It has the two plastic plugs. I drilled one out, and found that the plastic only goes in about a 1/8". Then I hit metal. Same for the other plug.
Well if you look at the construction of the thing, you have the steel round main housing, and the aluminum pinion housing (with drivers side bushing flange) casting that is "Attached" to the steel housing. If you look in from the end, you can see the steel center inside the aluminum part. What do you suppose keeps them together and prevents the two from rotating?
Well friction for sure, but those two holes have steel pins in them that lock the two together. The plastic just fills the remaining hole to the surface of the bushing housing.
To prove the pin theory, I used an 1/8 punch and whacked one of the holes until something emerged into the ID of the bore. It was not easy to move. I'm not convinced its a pin, but something emerges from the hole into the ID. Here ya go.
You may notice a little dimple sticking into the bore at 3:00 o'clock. This is from my punching. It sure as the heck isn't plastic.
So I don't really think the kind of plastic is important. Nor is filling the hole at all. Fill if you feel you must. If you drilled right through, I think you might want to put something back in there to keep the two parts from wanting to part ways or rotate.
Any ideas on pushing that knubbn back in? Rack ain't going back in with that thing sticking out like that....