Anyone who is interested in testing out the printed couplers, go ahead and email me off the group with your information and I'll see if I can get some sent out this coming week. It'll be one fret upper+lower per person, so enough to do six couplers total. That should be enough to test two cars worth and have spares in case you break something or mess it up.
I can't provide knuckles, balls, or springs. Those will have to be on your own.
Here's a quick video I took with a set of couplers I just assembled. The resin is Elegoo Gray and it is fully cured. I slightly enlarged the ball hole, and slightly enlarged the knuckle holes to more freely move on the pins. You can see the normal action with smooth coupling and uncoupling. Also note the strength when pressure is applied. I was pulling pretty hard here.