What 308 part is this?

Is that the valve that shuts off the overflow pipe if you roll the car upside down?

 

Looks like a check valve.

 

It's check valve E that allows air to be drawn into the system (if necessary), but prevents fuel vapors or liquid fuel from exiting the system:

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Attached is a copy of the 1977 North American 308GTB OM Addendum that has this figure for your reference.

US versions have a quasi-closed fuel system where any air+vapor that needs to come out (if things get pressurized from temperature changes) passes thru the charcoal canister carbon bed (to absorb the fuel vapors before the air is released out the hot air purge tube G), but there also has to be provision for letting air in (if necessary) so the fuel tanks don't get imploded (if a vacuum forms inside the tank as fuel is pumped out or due to temperature changes).

Euro versions just have a simple open tube connected to that nipple on the filler neck to let air in and let air+fuel vapor out as required.

 

Yup, it’s a one way valve.