I recently had a chance to visit the US Space and Rocket Center. While I was there, I saw a lunar module. It is very clear that the ascent portion is a mock-up, but the decent portion looks real, although it seems to be smaller than the one in the Smithsonian (Could just be because it is in a corner, the Smithsonian one you can walk around pretty good). I've seen mention on the internet that the descent stage was a test article, although I have carefully searched the lists and not seen any mention of this. Any idea as to what the origin of this test vehicle was? Thanks!
It's an engineering test mockup.
This model incorporates an engineering model of the Descent Stage (used at MSFC, designers of the Lunar Rover, for studies and fit checks) with a fabricated Ascent stage (note the lack of detail in the hatchway area compared to some other LM images)