Comment by Agraillo
9 hours ago
I like challenges like this. First, the edit that introduced the "five-seven ascii" is [1] (2010) by Pete142 with the explanation "add a name for the PDP-6/10 character-packing convention". The user Pete142 cites his web page www.pwilson.net that no longer serves his content. Sure it can be accessed with archive.org and from the resume the earliest year mentioned is 1986 ( MS-DOS/ASM/C drivers Technical Leader: ...). I suspect that he himself might have use the term when working and probably this jargon word/phrase didn't survive to a reliable book/research.
[1] https://en.wikipedia.org/w/index.php?title=36-bit_computing&...
You do better with a search for "PDP-10 packed ascii". In point of fact the PDP-10 had explicit instructions for managing strings of 7-bit ascii characters like this.