The 218 is a dual inverter and slew module.
This description is from the Series 3000 Modular Synthesizer Instructions Manual.
This reference diagram shows the values of the parts.
This image shows the inversion and positive offset of a triangle wave.
Slew only works on positive inputs. This image shows the maximum slew of about 3.5 S. The largest value I could find 1in a quality 16mm potentiometer was 1M. I could have increased the timing capacitor to compensate but I thought 3.5S was a reasonable maximum. The minimum slew is about 4 mS.