At one time, I worked for a residential builder and the draftsperson's table was equipped with something very similar to this. The round knob was to adjust for angles when creating renderings and to guide the mechanism across the paper.
Eh, used to manage a large art & drafting supply, still know way more about this stuff than I should... ~Cheryl
Wow - amazing !! I did look and look and gave up - but PEOPLE knowledge better than Google eh. One for the people xx