from The Century Dictionary.
- noun A design of plug upon which a thread is cut and which is screwed into the hole to be plugged. The plug is usually tapered three quarters of an inch to the foot or one in sixteen, and carries a squared end to receive a wrench by which it is screwed in to refusal.
Sorry, no etymologies found.
By means of a screw-plug passing through the bottom and entering the face of the hub the heater may be applied and removed in the course of five minutes, an important fact, as will be seen hereafter.
The screw-plug tubes, of course, were easily put in.