Rudolph’s Planar is a Double-Gauss: two almost symmetric halves around a stop, each a crown–flint pair. The symmetry cancels a lot of off-axis colour and distortion. Zeiss sold it as a fast anastigmat; later every camera company grew a version.
The name is the flat field, not a plane of glass. On the bench the stop sits in the middle and the meter sits where the plate would.