Lester Caine wrote:
The simple question that the provision of a version number allows one to answer is "what version of tz data was used to create the material being published".
Unfortunately that's not how things work. In practice the version number often does not provide that information; it merely provides a string that is useless or (worse) wrong. An example of this is the version string in the latest version of Fedora, which is merely "unknown". In practice there is no guarantee that a version string will be mappable to the data that it represents. There's not even a guarantee that differing data will have differing version strings.
there has to be some way to identify just what WAS used
In practice the version string does not suffice for that. You can use 'zdump -i', though.
ETags ONLY work for the person who originally normalised the data!
Sure, just as version strings are distributor-specific. ETags are no worse than version strings in this respect.