Hello forum. Newbie here. I recently grabbed up a 1981 XJ550RH SECA - one owner 12K miles on it. I am in the process of building - what a hope turns out to be a cool little cafe style bike. The gas tank is missing all the hardware. I see where the gas cap and petcock will go and have ordered the parts but what is the other hole for? What item goes there. I do not see it on any diagrams. Thanks in advance.