I didn't see anyone else mention it thus far. Perhaps you want to add the "magnet on a stick" check. If your plenum has been replaced with the popular aluminum part, a magnet will not stick to it. If the magnet sticks, it's probably the OEM part.
I like those flexible reaching tools with the magnet on the end. You know the one, the one you reach for when you drop that socket into a tight space and you can't get your fingers in the space to retrieve it.