This is the old style vented gas cap. They used to be available in hundreds of styles and shapes. Now they are hard to find. In fact this is the only one, other than the round domed right side fatbob thing I was running. It had a valve that protruded into the tank, and would siphon gasoline out if you fill the tank too full.
This cap is completely flat on the bottom, as it should be. The sproket styling is OK, but wouldn't be my first choice. It fits tight, and so far the gasoline is staying in the tank. The only downside as far as fit, I wish it wend down over the filler hole, and fit closer to the tank. It kind of sticks up.