This is the lid for the American Models Trainmaster draft gear. Sorry they are individually priced but 2 are needed. I LOST the Solidworks desgin files for the draft gear and the lid. Since I made 3 of these locos I am not making any more so no interest in recreating the SLSPRT file. But the STL file survived and is what is really needed to print anyway. The first photo is from when I had a Form1 printer and made them myself. The part was thick. See the below link for the Shapeways printed parts I used in a later project. They fit very well.
More info is here
https://www.lanestrains.com/Reading_Trainmaster.htm