I'd say TripOS.  There is some surce fragments but I never could get any BCPL to cross build anything.

Also Mach 1.x to 2.5

Sent from my Windows 10 Nokia Lumia 1520

From: Noel Chiappa
Sent: Sunday, 7 October 2018 9:05 AM
To: tuhs@minnie.tuhs.org
Cc: jnc@mercury.lcs.mit.edu
Subject: Re: [TUHS] Unix source code archive in the news

    > From: "Nelson H. F. Beebe"

    > a goal of a triplicated complete archive of the world's software
    > history, including both open source and proprietary code.  They report
    > holding 200TB of data already, covering 80 million code projects.

I should ask them if they have a copy of MERT!

Now that we have Multics, ITS, PDP-7 UNIX and UNIX V0, etc MERT (which may
have been the first micro-kernel - although perhaps THE gets that palm) is
perhaps the most significant 'missing' OS.

If not, what am I missing? The Berkeley Genie OS for the SDS? (Dunno if that's
been saved.) The THE system? (Ditto - although I know someone has saved the
last X8.) The Atlas OS?

     Noel