a) - thanks Ned, for the kind words.

b) - the proposed (change to the tag) is "AIX.VRTL.YYWW.SZ".
"AIX" - in caps, to distinguish from current tag starting as "aix"
VRTL - 4 digit number, one digit for Version, one digit as Revision, and two digits for Technology Level
YYWW - builddata with two digits for year (7 becomes 07) and two digits for week of year.
SZ - 32, or 64 bit size of executable (or pointer).

The builddate is crucial as system updates are possible, in oslevel -s terms, from 6100-09-11-1810 to 7100-05-02-1810 but not from 6100-09-12-1846 to 7100-05-02-1810. Using the proposed PEP425 tag the VRTL-YYWW numbers here would be: 6109-1810, 6109-1846 and 7105-1810.

The build-date of the executable is essential, as it specifies the lowest date of any AIX release that it is supported on. This is something CPython must provide, as it does for other platforms, in the form of sysconfig.get_config_var("AIX_BUILDDATE"). The VRTL values of the executable are already available via sysconfig.get_config_var("BUILD_GNU_TYPE") while SZ is typically determined from sys.maxsize.

Happy to answer additional questions - and write additional .rst documentation as desired.

e.g., an