I don't think that area that is missing is for an elbow. It is more like a tee with a spot for a pressure gauge and a sample port. As for your outlet. There should be a pressure gauge and sample port on that side as well. Most plate and frame filters I've seen have the outlet coming out from the top not the bottom like in this pic. The inlet normally goes in the bottom. It looks to me like you have a piece of equipment here that was robed for parts and somebody put it back together wrong. Private message me if you want pics of my set up.