This prototype field gun car was found at York in April 1999! They're still out there!
Built as a scale car using the platform from a side dump car and a scale caboose frame, it indicates that Marx was considering military scale trains. It makes you wonder if they made any others! Have you seen any?

Here is a picture of the coupler on this car. Have you ever seen one like it?
Trip and I have discussed this and here are our thoughts.
The bulge in the vertical part of the coupler was probably for a return spring which was later redesigned.
The shoe of the coupler differs from the ordinary in that it has two diagonal "skis". The production coupler has only one "ski" at the inside end of the coupler shoe. If you understand how the uncoupler works, you will see that this coupler would uncouple the cars at every pass, regardless of direction. This has led us to believe that the uncoupling track was originally designed to operate manually much like the AF link coupler.
The production design is much better!

Courtesy of Trip Riley