Walter, this cap is a locking twist-on type. At the open position, the movable spring-loaded steel tabs align with the tabs under the gas cap. These tabs are cast into the bottom of the cap near the rubber gasket and are 180 degrees from each other. When the cap is in your hand, the key will twist 90 degrees clockwise, which turns the spring-loaded steel tabs 90 degrees. The cap is now in locking position, and this is the only position that the key can be removed.
The problem is that the tabs cast into the cap just below the rubber gasket are too wide to drop into the slots in the tank's hole. Each tab must be narrowed maybe 2mm(.080") with a dremel tool. It is extremely labor-intensive to file the tabs down because the file can't get to the tabs easily under the gas cap. Another option is to widen the slots in the tank's hole, which is probably what I'd do.
Once the cast-in tabs clear the gas tank's slots, I should be able to press down to overcome the spring-loaded steel tabs, twist the cap 90 degrees and lock the cap.
This particular locking gas cap is not a bolt-on; it has to be modified to fit.