The "C" in "Ektanar C" is for (C)urved field. This lens is used to correct for the curvature of a slide that has "popped" on account of the heat from the projection lamp. "Popping" is a normal thing when projecting slides that are in cardboard mounts, or glassless plastic mounts. It is due to the unequal expansion, from heat, of the emulsion and non-emulsion sides of the film. Glass mounting prevents this. For other mounts, a "C" lens may help to maintain focus across the field.
May the light be with you.