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

[Jonathan Wakely]

On Thu, 2 May 2024 at 23:25, brian.inglis--- via tz <tz at iana.org> wrote:
>
> On 2024-05-02 12:35, Paul Eggert via tz wrote:
> > 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.
>
> I would expect it to default to "S" to allow "%s" in zones without rules.

Wouldn't that be wrong for several European countries using CE%sT
where the LETTER/S for standard time is "-" and for daylight savings
time is "S"?