https://groups.io/g/Hercules-OS380
The list was once for people extending the FOSS Hercules emulator and the early IBM OS/360 source code (available, but not technically FOSS) to make an emulator for an updated mainframe that never existed but could have more memory than the real things. Sounds crazy but it has some practical uses: IBM did do this later, but its OSes from that era are not FOSS or free: they are very expensive.
The author of PDOS seemed to feel that PCs ought to have descended from miniaturised mainframes, not from a scaled-up Intel 8080. He wanted to write a Unix-like OS but for mainframes, and port it to the PC.
It was very strange and confusing, and he did not explain it well at all. He seemed not to know (or care) about a lot of the history of computers, OS development or anything else: he seemed to feel he knew better and didn't need to know.
In the end, the group was almost all his posts, and I left it, unable to cope with the torrent.