It sounds like the coupler you have is similar to one I found in one of my gondolas, mixed in with a scrap load. I had a bunch of these, but many years ago, gave them to somebody at the hobby shop who used them on his trains. This one got left behind. The uncoupling piece has been removed, but I recall it as a flat piece of brass with a curve in it, similar to the uncoupling pin on a Kadee, but with a gentler curve. If I recall correctly, there was a notch or two in the inner face of the curve, too. You can see the slot in the head of the coupler where the uncoupling pin was located, especially in the second view. The shank is very similar to a Kadee #4, which was at one time Kadee's standard. Here's a look, with a Kadee #5 for comparison:
For some reason, I'd also thought that these were Baker couplers, but as I'm typing, Mantua springs to mind. Ray?? :-D
Wayne