HH stands for "Hansestadt". Hanse was a kind of trading contact, you can say one of the first "free trading zones" in the middle age. (or around). There are some cities in Germany, some in Sweden and Denmark - but don´t harm me, if I forgot something. In Germany there were (and wear in their car reg. still today: Bremen (HB), Hamburg (HH), Rostock (HRO), Greifswald (HGW), Wismar (HWI), Lübeck (HL).
In fact Hamburg of course is not the "Hauptstadt/capital" of Germany - it´s only the second largest city. But you could call it Hauptstadt, too, because the "Free and Hanseatic City of hamburg" is one of the sixteen federal states - so it is it´s own "Hauptstadt/capital". The two other "city-states" are Berlin (the real federal "Hauptstadt") and Bremen.
Sorry, little trip into Germany - not in flying, but perhaps it answers some questions.
The street the Airbuses have to cross at the Finkenwerder plant, where the pic was taken, is called Neß-Hauptdeich.
Regards from Munich