Which end is up

My lower door assembly looks like the upper door assembly, and vice versa. The door doesn't appear to be on upside down, but considering the hack we had install it, anything is possible, and the thing has never really worked right. Would there be any other reason why this would be? I'm really looking just for the plastic wheel that fits into the assembly, because one of ours is broken.

Sure enough. I took a closer look, and the moron did install the door upside down. A quick 180, and a flip of the handle from left to right, and the thing works like charm. Should have checked this out earlier, since the guy screwed up just about everything else he touched too.