ARCH OF JANUS
The Arch of Janus is being used as a wild card. It is known that the four way arch that is still partially standing today (pictured) replaced an earlier arch, it is this earlier structure that we are using as our wild card.
Picture from Platner and Ashby on-line Topography of Ancient Rome
The arch is said to have probably marked the separation of the Forum Boarium and the Velabrum.