This is a Fill It Fueling System from Dubro.

FEATURES: 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.

INCLUDES: Housing
Four Fill It Fuel Line Plugs
Clunk
Fuel Tube for third line pick-up

REQUIRES: CA Glue or Epoxy
Fuel Tubing

SPECS: Fuselage Mount:
Inner Diameter: 5/8" (16mm)
Outer Diameter: 7/16" (11mm)