Looks like the end opposite the hole has broken off?

Yes it is definitely the carriage frame stop.  It should be secured by a long screw (which also holds the plate on that has the scale on it, just as shown in the illustration).

That diagram came from a standard typewriter repair manual pdf, which can be found towards the bottom of the Classic Typewriter Page, here, under service manuals. It is called:

Ames Typewriter Mechanical Training Manual, 1945, vol. 1, for Remington, Underwood carriage-shift, Royal, L.C. Smith, and Woodstock standard typewriters, by Clarence LeRoy Jones

It may be a different model than yours (I can't remember which Underwood was covered in that manual), but should share many of the mechanics.