Plug. PEEK, 1/4-28, ARE-Applied Research brand. 1 EA.
Knurled for a tight grip and also has "flat" surfaces for using a wrench to quarter tighten for best seal or to loosen if plug gets stuck.
Screw thread for 1/4 - 28" holes.
Use with Delivery Cap 58820-06, to prevent oxygen from entering the bottle while one of the holes are not in use.