Karolnka, I do know some things about chocolate (I am Bulgarian living in Switzerland :)
It seems to me that the bar you bought were either expired or not stored properly. I have eaten many many LZ chocolates and they do melt quite nicely.
LZ is indeed sold as aero bar, it says "аеро шоколад" on the back side, as far as I recall. It is marketed as "chocolate with atmosphere" - light and airy.
LZ is the IATA (International Air Transport Association) code for Balkan Bulgarian Airlines - government-owned national carrier between 1946 and 2002.
Why LZ and not BG, you may be asking yourself.
LZ comes from the name Lazarov - prof. Tsvean Lazarov is Bulgarian airplane engineer and constructor; he set up the first researches on air dynamics in Bulgaria and constructed several training airplanes  - DAR and LAZ.