Jeff, the key is literally that, finding ut what kind of fuel caps were in use, where they were located, and what type of key was needed to open them. Being an aircraft, I would presume that the fillers were flush with the wings/fuselage and would need to have some type of key to open them. On my old boat I had a key that was basically a small handle for leverage that had two knubs protruding from the bottom that inserted into two holes in the filler cap. I'm thinking that a 'T' shape in a filler cap might be needed to give one a bit more to grip with the key to seal the filler tightly. I, too, see a 'T' within a circle. I've done lots of diving around coral and haven't yet seen anything like that. LTM, who hated losing her key. -John