Thank you very much Lordflasheart for that research, and that made me dig in the attic for my original reference, which is (for those who have it) Propliner magazine No 29 (winter 1986), article “Over the Top of the World – SAS”, all about their pioneering DC7C Trans-Arctic flights, which includes a lengthy section “The Problems of Polar Navigation”.
This describes ‘three main obstacles being the existence of the Magnetic North Pole, the Polar Twilight, and the disappearance of direction at the Pole’. SAS were indeed pioneer users of the Polarised Sky Compass developed by the Kollsman Instrument Company (still in business, I see, in New Hampshire, although the products have moved way on). Magnetic North was handled by the Polar Path Gyro, which was a Bendix product, also applicable on transatlantic flights to New York, and “everywhere-south” issue at the Pole by a Polar Grid Chart, which was an RCAF concept.