The reason I asked is, the overflow tank (the long one) does not normally have a pressure cap. Being open to the atmosphere in order to work as designed, it only gets a similar looking non-sealing cover to prevent splashing overflow coolant. The short swirl tank has the only pressure cap. Unless the car has a non-stock cooling system routing, or possibly the short tank is mounted low and that AN fitting connects to a hose going to a remote rad-cap somewhere convenient.
Which is how my homebuilt system works. I fabricated a duplicate outside-fill cap under a hinged cover in the right gill, for access to a remotely mounted radiator cap connected to a low mount swirl tank. Our car is an L model w/stock outside left-side fueling, so no need to open the deck lid for either refueling OR filling the radiator. Been rigged this way since the '90s and is very convenient. FWIW, my stock water temp sensor is mounted in the block for more accuracy, like millions of front-engine Fords.