The moving finger stop was a feature of the Trimline dial phone.[1] It allows making the dial slightly smaller. On most dial phones, there's a large gap between the 1 and 0 holes. On Trimlines, and this new thing, there is no gap. Without that moving finger stop, you get a dial handset like this AT&T prototype: [2] That was called "the Schmoo".[3] Huge bulge in the middle of the handset.
The moving finger stop also appeared in 1960s versions of the Lineman's Test Set (the "butt set", usually hung from a tool belt), where the dial was on the back of the earpiece.
etymologically, the term Schmoo or Shmoo (typically based on the characteristic shape) originates with the cartoon character popularized by Al Capp: https://en.wikipedia.org/wiki/Shmoo
A favorite example is the shape of a budding haploid yeast cell, known in the literature as a shmoo (though as that wikipedia page will indicate, there are a lot of other delightful schmoo/shmoo eponyms): https://web.archive.org/web/20071111122821/http://discoverma...
A favorite example is the shape of a budding haploid yeast cell, known in the literature as a shmoo (though as that wikipedia page will indicate, there are a lot of other delightful schmoo/shmoo eponyms): https://web.archive.org/web/20071111122821/http://discoverma...