When I switched scales from N to S, I quickly became aware of the Sergent Engineering couplers. However, by then Frank Sergent had sold out of the S-scale couplers and was in the process of redesigning his coupler range of products, so he wasn't going to make any more of the "old" design. I was able to buy three of the old S-scale couplers from someone on the (former) Yahoo S-scale group mailing list. Shown here are the parts for the three couplers. The two halves of the coupler body are the larger parts, and the coupler faces are near the center/top. Also included are metal balls and springs.
I am documenting these purely for historical reasons.
Then the coupler face can be installed. There is a small hole in the coupler body into which the coupler face's rod is to be inserted. Position the coupler face as shown in the photo.
You can then carefully place the top half of the coupler body on the assembly, again inserting the coupler face's rod into the hole of the upper half.
I then used superglue to glue the body halves together. I carefully held the coupler in one hand while applying a drop near the gap between the two body halves near the back. Capillary action takes over and glues them almost instantaneously. I made sure to keep the glue away from the coupler face and the area where the metal ball resides. This photo shows the coupler's closed position.
I then proceeded to install one coupler on my test car. It currently has Kadee S-scale couplers installed. I removed the coupler from the car and removed the coupler from the draft gear box itself. The Sergent coupler fits perfectly. I didn't install the spring, because I didn't see the purpose of it in this application. I then learned that the spring isn't intended to center the coupler (like the Kadee's do), but rather to provide slight resistance to movement, so that they will stay in the position that you put them (just like in the prototype). The whole assembly fits perfectly inside the Kadee coupler box. Note that upon installation, if the screw is too tight, the coupler won't move freely, so I tightened it all the way and then backed off the screw by a quarter turn.
This photo shows the test car next to a S-Helper Service coupler on the NW2 engine. I still need to cut off the protruding coupler face rod, but as you can see, the Sergent coupler is at the correct height and looks fantastic. Note that the Sergent couplers are not designed to be compatible with Kadee or other S-scale or HO-scale couplers. This is done on purpose, because the Sergent couplers are supposed to be a replica of the real thing. My experience was that you can couple them together by lifting the car with the Sergent coupler onto the Kadee of SHS coupler. So, it is possible to build a semi-permanently coupled train by hand, but you cannot couple or uncouple them from each other, unless you lift one of the cars. However, you can have a conversion car, where one side has a Kadee-compatible coupler, and the other side has the Sergent coupler.