This could be set up as uniflow.  The line you call the fill line is actually the vent line, the vent line is a vent line used for filling only.  Make sure the vent line (the other line with a clunk) is shorter than the pick up line.  When you fill it you unplug the fill vent and pump fuel till fuel runs out of it.  Fuel may come out the pick up line and flood the carb, especially if the carb is not at idle, so this is often plugged during fill up.  Although on many of my  uniflow setups it is just enough to prime the engine.