Interesting. Thanks Uwe!
Looks like this class of machines is discussed on the same page that Pete pointed me at the other day (Special Purpose Office Typewriters (officemuseum.com)), and includes how the maker of one such machine, Elliot & Hatch Typewriter Co, merged with a competitor, Fisher Book-Typewriter Co, which explains the Underwood connection.
Curious design: the majority of the typewriter is also the carriage as it looks like it slides along a stationary mount.
I wonder how they line it up on the page. A few more pictures here (Underwood Typewriter [History, Value, Models] - A Complete Guide (txantiquemall.com)) but none show any crosshairs or alignment guide. These and the officemuseum photo make it look like there may have been wheels at the back (which the Breker machine is lacking), perhaps to scoot the whole thing along as a line advance?