Locking Fuel Cap

[xz3kg7]

Does anyone lock their fuel tank? I just started parking my coach
someplace besides my house and with $1000 worth of fuel in the tank I
want to lock it.

My fuel cap (the big brass Bluebird one) has a lock on it, but I can't
tell how it is supposed to work. It has a half-moon piece on the back
that turns... but there is nothing for it to grab. Not to mention,
the piece turns a half turn, but to get the key out, you have to turn
it back.

How does yours work? (Also, if there is another post on this, I could
not find it.)

Thanks,
Alan 87FC35