[makomk]

As far as I can tell the HDC-402 wasn't even the Viking computer that got reused on Voyager anyway - it was something NASA developed in-house from the orbiter side of the Viking mission and that presumably has even less documentation available.

[bilegeek]

You're confusing the 24-bit HDC-402 on the lander with the custom 18-bit computer on the orbiter (p.164), which had it's origins in the sequencer for the Mariner missions (p.152-154).

Though given the apparent level of NASA involvement in the 402's design, and the lack of evidence I can find for use outside of NASA, it might as well be called custom.