This is a Fill It Fueling System from Dubro.
Four Fill It Fuel Line Plugs
Fuel Tube for third line pick-up
One fueling system that can be used with four different tubing sizes:
1/16", 3/32", 1/8" and 5/32" ID
Fill It Plug has handy knob for easy removal of fuel line to fill or
drain fuel tank.
Ideal for cowled engines.
Lightweight, no leaks.
Can be used with gasoline or glow fuel.
Inner Diameter: 5/8" (16mm)
Outer Diameter: 7/16" (11mm)
CA Glue or Epoxy