Many Mk1-Mk2 classic Minis that came to the USA were fitted with twin fuel tanks. This pipe interconnects the two tanks so they drain simultaneously, maintaining an accurate fuel level reading. Ref: 21A2331.
The only correct braided/rubber hose for the 5/16" diameter of this pipe is 21A2332, sold separately.
For a complete twin tank installation kit, see SIE0012.