Trammell HudsonSlide rule fans: have you ever seen one with two cursors and a second sliding rule? This odd sliderule illustration was on the cover of Astounding Science Fiction, May 1951 https://archive.org/details/Astounding_v47n03_1951-05_Sam_Hall/mode/2up
Lee Holmes 
@th It kinda looks like that rule is for unit conversions? So maybe to support a->b with regular C/D scales and multiple b->c conversions where you don't want to lose your original calculations?
@th Oh, that magazine is sci-fi. Perhaps it's just invented to look fancier and more complicated.