> Arguably ancient PDP-10 operating systems like ITS, WAITS, TENEX were
> somewhat "open" and "free", but it's not a clear cut case.

The open source movement was a revival of the old days of SHARE and other
user groups.

SAP, the SHARE assembly program for the IBM 704, was freely available--with
source code--to all members of the SHARE user group. I am not aware of any
restrictions on redistribution.

Other more specialized programs were also freely available through SHARE. In
particular, Fortran formatted IO was adopted directly from a SHARE program
written by Roy Nutt (who also wrote SAP and helped write Fortran I).

Bell Labs freely distributed the BESYS operating system for the IBM 704.
At the time (1958) no operating system was available from IBM.

IBM provided source code for the Fortran II compiler. In the
fashion of the time, I spent a memorable all-night session with
that code at hand, finding and fixing a bizarre bug (a computed GOTO
bombed if the number of branches was 74 mod 75) with a bizarre cause
(the code changed the index-register field in certain instructions on the
fly--inconsistently). And there was no operating system to help, because
BESYS swapped itself out to make room for the compiler.

Doug