I'll add that POSIX, as we know it today (and really since at least 2000) is a collaboration between The Open Group, IEEE Std 1003.1-XXXX and ISO/IEC 9945:YYYY (collectively known as the Austin Group, though why "Austin" I cannot say).

Because IBM hosted the first meeting in Austin, at their Unix development center.  Check out the footnotes at https://www.filibeto.org/sun/lib/standards/susv3/frontmatter/preface.html