(Most of) the source was distributed with the operating system binaries, as microfiche.

I spent a few days peering at an old 'fiche reader figuring out how VMS stored user-defined account text in the process control block. We were doing chargeback you see...

The fun bit was the explanation of how a VAX booted VMS. The Cheshire cat was the mascot: the CPU was set up to have real and virtual addresses line up, then the VM bit was turned on, and...presto, no cat.