The Sholes and Glidden typewriter (sometimes called the Remington No. 1) was the first successful typewriter ever brought to market (in 1873), and the forerunner of most other successful typewriters. The unidentified key was, as far as I can tell, on this model and only this model. It was gone on the Remington No. 2 introduced in 1878, never to appear again (in this form), and as far as I know never found on competitors either.
I love stuff like this.