Yes, definitely the cathode. It is a common thing on early solid state diodes that the red terminal is the cathode. It seems designed to confuse people, but I suspect the thinking was that if it were a PSU, the cathode of the rectifier is connected to the positive connection of the smoothing capacitor...
I guess it is no more confusing than modern electrolytics having a stripe indicating negative whilst tantalums have a stripe for positive
cheers
Andy