
The short side of the plug should face up in the assembly and the long side should be down against the cast stop in the body.

As you can see in the picture above, the long side of the plug is facing down and resting on the stop on the upstream side of the fitting. The short side is facing up towards the top of the fitting.
Note: If the plug is installed with the long side up, it will strike the seat plate and prevent it from rolling into the closed position.