[tz] Why is "AEST" the abbreviation for Australia/Sydney in 1900?

[Paul Eggert]

On 2024-04-30 16:21, Arthur David Olson via tz wrote:

> For a given zone, each line describing the zone except for the last ends
> with an until time. The creation of the transition time entry for the until
> time is deferred until the following zone line has been completely
> processed. (It happens at the bottom of the giant for loop in outzone.)
> That deferral means that the time zone abbreviations in use have been
> computed, so the appropriate abbreviation can be applied to the until time.

OK, but this wouldn't address the issue of what happens with the last 
zone line, as that lacks an until time. For example:

  Rule Aus 1917 only - Jan 1 2:00 1:00 D
  Rule Aus 1917 only - Mar 1 2:00 0    S
  Zone Australia/Sydney 10:00 Aus AE%sT

zic generates a TZif file where time type 0 (the time type before 1917) 
uses the abbreviation "AEST" - but where did that "S" come from? The 
documentation doesn't say. I think this is Jonathan's main point.

When computing %s for timestamps that come before the earliest rule, zic 
uses the LETTER/S field of the earliest rule that specifies standard 
time. I installed the attached to try to document this.
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