Wow, didn't know that the Post Yellow is named "Ginstergelb". Here everybody is knowing it as "Postgelb".

Ok, to be precise Umlaute are only given sometimes in the first letter group which is derived from the town or region names and is fix given by a list.

But in the second free combined letter group they are forbidden. That english wiki article is (not only there) somewhat unprecise. The german wiki article is much more exact and seems to be on newest stand with for example changes from 2011.

Die Erkennungsnummer besteht aus:

• einem oder zwei Buchstaben (möglich sind alle deutschen Großbuchstaben außer den Umlauten „Ä“, „Ö“ und „Ü“) (früher gelegentlich per Dienststellen-Kontingent der Zulassungsstellen-Niederlassung vergeben). Teilweise sind nicht alle Kombinationen möglich (siehe auch den Abschnitt Verschiedenes). Bei Behördenkennzeichen entfällt dieser Bereich;
• einer bis vier (abweichend bis zu sechs) Ziffern. Bis zu sechs Ziffern werden vergeben, wenn es die einzigen Unterscheidungsmerkmale innerhalb eines Gebietes sind. Das kommt bei Behördenkennzeichen vor, da hier die Buchstaben fehlen. Bei der Vergabe der Zahlen von Behörden sollen zur Unterscheidung neuerdings an die einzelnen Behörden Kennzeichen mit Nummern aus verschiedenen Bereichen zugeteilt werden.

Also for example this text from general start of the english article is unprecise:

Quote - Number plates can be bought which are valid all year round or between 2 to 11 months within any 12 months. This allows to save money on vehicle usually driven in particular part of the year, eg. motorbikes. As of 2007, buying new number plates normally costs around €30 and €10 to €40 for fees for de-registering the old plates and registering the new ones.

How long a number plate is valid in the year is something of a insignificant information. The normal way is that the number is valid whole year and its not bought every year. You get the number with first registration of the car and then have to pay your tax year by year. If you don't pay the tax not the number becomes obsolete but the complete authorization for the car is out of order. A once given number stay with the car as long as it's not permanently stationed in a other town or region with a other first letter group.

So the english wiki this time is not best information source I guess.

I'm not sure but if the volkseigenes Gelb is standing for the easter VEB Kraftdroschken (Taxi) didn't they have the black-white checked line around the car until the end. Though for that El Benzo only a theoretical question cause a) to young for GDR and b) everytime to expensive for them. They drove Trabant or as luxury version a Wartburg.