I was always interesting in registration issue.
The best one that I saw is :
LZ BTZ
at first glimpse nothing interested but further analysis:
LZ- Two letters designated country. In this case Bulgaria / for those less familiar with current situation in EU- Newest member of EU !
The third letter B- usually designates that a/c is not in commercial traffic i.e. state a/c carrying VIP . In this case VIP Tu-154
TZ- abrevation for TODOR ZIVKOV- famous Bulgarian Head of State during their communist era. Friend of Chausesku, Stalin, Hruscov, Erih Honeker and other communist brains from Cold War era.
It was very logic in that time to give registration in such manner.
The joke is today.
this a/c still has the same registration, carrying now democratic and European minded politicians from Bulgaria.