This Is A Fuel Filler Valve from Sullivan.
For easy filling and defueling of a model fuel tank from outside the model without disconnecting any fuel lines. Simple check ball design works with any glow fuel, diesel fuel, gasoline or smoke fuel.
Can be located in any convenient place in the model. Large fuel passages are suitable for any engine size. No special probe is required. It is white plastic, and also comes with 2 nuts and 2 metal ties for attaching the fuel lines.