As Wynn said, the vents are for filling and overflow. The top one is used for filling and the open vent (or muffler pressure) and is hooked, naturally, with fuel tubing to the uniflow or top vent on a standard tank. The overflow can actually be put anywhere convenient with uniflow. I have gone to running it out the bottom to keep raw fuel off the plane. And it is capped when the tank is full. On Classic ships, I turn the overflow down for the same purpose.