I think Brian was right in the first instance, in the PT-6 the Ng = N1, Np = N2. My understanding is it's all about which compressor/spool sees the airflow through the "jet engine" pump first. In a typical Turbofan the LP compressor or large forward fan sees the airflow first so is N1, then through N2 etc. In a Turbo-prop the prop is not part of the pump, so the first part of the engine that compresses air is the gas generator (Ng) making it N1. The airflow doesn't reach the prop driving part until last in the turbine stage. The PT-6 though is different to same other turbo-shafts as the Np shaft is separate to the Ng shaft, unlike a CT-7 for instance where the prop shaft is more conventionally placed through the Ng shaft and airflow goes in the correct direction.