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I tried the search with W *** UA, than W *** UB and so on until W *** UZ. With W *** UH there are 6 results, all dealer plates. The other searches had 0 results, even W *** US had 0 results:
The oldest S *** T* spot is from 2016, the oldest S *** U* spot is from 2018, so the T-series in Salzburg was between 2016 and 2018.
If TS is a special dealer serie, there should be only dealer plates with the serie S***TS serie, but i could find regular plates in this serie as well:
I think Salzburg gives a few weeks during the TR and TS serie dealer plates with regular format. It's not the only muncipal, I could also find an example from Spital an der Drau between normal SP *** AK plates:
In Vienna dealer and export plates have the format W12ABC, Temporary plates have W1ABC,W12AB or W123A. Motorcycle and moped plates have W1ABC or W12ABC.
The format W123UH and S123ZZ seems to be only for dealer plates.
W *** US is uploaded but you sadly can't see it because it was uploaded as hidden, here is a screenshot:

Interesting that Salzburg gave out specialty and regular series plates during TR and TS. Theoretically these examples could have been initially given out as regular series to a person who owns a dealership or to the dealership itself, and then at request reissued as a dealer plate. I know this is possible with vanity plates, you can see it by the example of the plate SP ROC 1, which first existed as dealer and was then reissued as a regular series plate. However, this wouldn't explain the export plates which were spotted in that series too, so this is really weird.
The SP 500 AK has the following, long and complicated explanation:
In Carinthian districts, the first format, which was XX 10 AA - XX 99 ZZ in all districts except Klagenfurt, was given out from 1990 until approximately 1999 and was the same for all plate types, meaning regular as well as all specialties like dealer, moped, export and the others. Around 1999, the second series released, which was XX 100 AA, and it was still the same for regular as well as specialty series, but only for a short time. I know this because there have been several spots of dealership and motorcycle plates with that series, but only early combinations, like the SP 500 AK plate. Probably from the mid-2000s, specialty plates recieved their own formats in Carinthia.
The formats in Vienna are really complicated. The ones you listed are only the current ones, right? For exmaple, with Export, there is also the W 5497 D plate uploaded. Vienna is also the only district which has a special format for two-row plates on cars, which is W 10 AAA since 1999 and W 1000 A from 1990-1999.