Terminal 16 gets earthed to earth the transformer cover and the transformer stack. Yes, PSU is power supply unit. The audio ground and the chassis earth connect at terminal 3. The ports on the schematic are placed in the most convenient place. For example, the PSU ground would more accurately be drawn as coming into the headphone jack, but for the purposes of the kit schematic it's more convenient to simply draw it as shown. Similarly the schematic shows separate ground wires for each channel going to the potentiometer, which is not reflected in the build. The Crack schematic is a little unique as it was drawn out formally long after the first amp or two were built, but it's certainly sufficiently representative to understand what's going on.
