Seminifty metal circular slide rule. Once you've worked out how to use one of these things, they can be awfully efficient. The rule is a single piece of metal, with rotating cursors. Roughly, the long cursor corresponds to the slide, the short to the cursor on a straight rule (surprise). Most calculations are made by setting one cursor (i.e. moving the slide), then moving the other cursor, which causes both cursors to move in tandem, keeping the angle between them constant. The reverse side has only one cursor because it's trig functions and fractional equivalents, which only need the single cursor.
There are some problems with this sort of rule; the worst is that there's a plethora of scales, none of which are labelled. It's a log-log rule, but you'd never guess that from peering at the scales. Also, it's really difficult to keep the angle between the cursors from changing when you're moving them in tandem; even minor changes in angle can introduce pretty significant errors.
It's probably not entirely proper to put this rule with the Dietzgen rules; Nowhere on the rule itself is it indicated that the rule was sold by Dietzgen. These rules were manufactured by Gilson and resold by a whole mess of other companies. I only know this one's a Dietzgen 'cause it says so on the manual. Given the design deficiencies in the rule, I'm not sure why so many people bought them, except that they were cheap to manufacture, and presumably to buy.