Thank goodness for people with weird hobbies. Arnold Betzwieser is a tax lawyer in Miltenberg, Germany. On the website for his law firm, alongside business info, he has a non-tax-matter-related section called "Small Exhibition: Interesting facts worth knowing about the historical development of office technology – when there were no computers."
"The 'Gritzner' was a typewriter specially developed for use on the drawing board, which was used in particular by architects, designers, technical draftsmen and similar professions for writing on blueprints and other technical drawings."
"The typewriter ran on a rail that could be moved like a ruler on the drafting table. Unlike normal typewriters, the "Gritzner" did not have a carriage, but wrote directly on the flat writing surface of the construction drawing in the upper lapel. The machine was therefore one of the 'flat typewriters'."
"With each keystroke, the machine moved one character width to the right on the rail above the paper. A long helical tension spring attached to the underside of the machine, attached to one long side of the rail and running via a deflection roller to the other side of the rail, served as the pulling mechanism (picture below)."