I was just looking through some NCR back-issues dating from 1976, and came across a letter from Eric Brockway replying to a query about the HPN frame prefixes. In the Oct/Nov issue, he states the prefix refered to Heat-treatment Pickle Normalized. It seems strange that they would make a special note of a production process in the frame number. But the late Eric Brockway was Managing Director for Douglas Sales and Service, so would have been in the position to know. As previously mentioned, they were issued as warrenty replacment frames.