This is a Modern Coupler Box, designed to accept the San Juan Car Co. #1501 AAR "E" Type Coupler.
THIS IS A PROTOTYPE: This has not been reviewed/tested yet - currently is up for sale so the person who requested it can do that - still - confidence is 'high'
I created two versions .. one with bolt holes and one with bolt heads. I really don't think the O scale 1-1/4" bolt heads will print well enough but .. who knows. We will have to see what the print looks like. The one with the holes - they are .020" holes so Tichy #8142 1.25 nuts should slip in or will at minimum provide a good guide hole for the drill)