It wasn’t a door. It was a station on the fuselage where a door could optionally be installed.
Actually, I never called it a door in my previous posts, but it actually by definition a door. It has two large hinges on the bottom along with straps, springs, a guide track and rollers along with four bolts and pins that hold it shut. It's official name was the "left mid-exit door".
I did not realize that this one was not configured as a normal emergency exit. It could have easily been re-configured as an emergency exit any time that the owner of the aircraft decided to add more seats. And because it was easily identified as an opening on the outside of the aircraft... emergency personnel could open it without causing damage to the fuselage if necessary.
If you are interested there is a very thorough video at The Boeing 737 Technical Channel: