[piltdownman]

There's IIGS 6.0.1 Source Code available, but even more interesting is the source for A/UX, which is UNIX System V 2.2 specially adapted to have the System 7 graphical interface running as a UNIX application.

[Lammy]

Pardon my redditism but I like your extremely relevant username

The ROM source has several change logs mentioning “PDM”