From the 604 Operating Manual:
FUEL TANK SYSTEM (CONT'D)
Components and Operation
Main Tank System
The left and right main tanks are integral-type fuel tanks (wet wings) that supply fuel to their
respective engines via two collector tanks. Flapper valves in the wing ribs prevent fuel from
surging toward the wing tips. The main tanks may be refueled by pressure or gravity. The APU’s
normal fuel supply is from the right main tank.
Collector Tanks
Two collector tanks, situated at the lowest point of the center auxiliary tank, are considered
part of the main tank system.
The 601 was the same, but that's not relevant to this thread.