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.

[1] https://en.wikipedia.org/wiki/Trimline_telephone

[2] https://www.bellsystemmemorial.com/images/dl/misc/schmoo.jpg

[3] https://www.bellsystemmemorial.com/telephones-trimline-artic...