The leading early American OO manufacturer was Nason Railways (more here). With a complete line on the market in 1934, a very necessary item to produce was a coupler.

Being first, they did not need to make a design that was compatible with any other maker in any way. But they did need to make something that would satisfy their customers! As with their other early products, Nason couplers are sand-cast in bronze. They are sand cast, so there are some variations depending on the pour of bronze. But every coupler I have seen has been dressed with a file to a fairly consistent standard, most likely at the factory.

As can be seen in this first photo, the Nason coupler is a bit clunky looking, but Lionel and Scale-Craft couplers are compatible.

This second photo is of how a vintage, scratchbuilt stock car was set up with Nason couplers and other Nason parts. The small shank allows the builder to use a small nail to attach the coupler. These couplers are not seen often but are a neat touch to look for on early, vintage models.