pdp-7 Unix
Unix version 0 was written in 1963 by Ken Thompson, on a PDP-7. Recently, the source code code Unix V0 has been discovered, and you can read it here, as pdf scans of printouts. You can read about the discovery, and the effort to boot Unix V0 on a real PDP-7 here. The project home page is here.
I got interested in PDP-7 unix, and then in PDP-7 assembler. Eventually, I wrote an Antlr4 grammar to parse PDP-7 assembler files in the original as format that Thompson wrote them here. The resulting grammar is here.