The height in the corridors was determined by the height that the Yorks and Hastings etc could fly without oxygen. It was not however as low as quoted above.
However Rule 2 was the need to achieve separation from notified Soviet exercises. In this instance, one would fly below the usual minimum corridior height.
On one occasion we had to fly at about 3000 feet over, IIRC Mahwinkel, the largest helicopter base in the DDR. I did not see so much as a single GAZ let alone one of more than 100 choppers.