Quick identification..... this measures approximately 13mm O.D. across the outside of the NONE tapered thread
(( Easily mistaken as the American NPT tapered thread found in some Harley tanks, that are simula threads per inch, but are approximately 13.5mm at the larger end ))

Used to create a reserve by having a level pipe inside... so when used with its matching other tap ( without a level pipe inside ) it leaves you a quantity of fuel as reserve... if clearance permiting for filter length and pipe.