This is UAZ 469. Again a "hard core" Russian machine that was produced virtually unchanged for like 40-50 years, if not more
These days they call it "UAZ Hunter":
http://www.uaz.ru/eng/models/hunter/index.htm
And hunters around here love them indeed.
I doubt they were running on natural gas (CNG), because tank bottles for it are quite heavy and usually with lower volume, due to the high pressure required to keep the gas liquid in them.
More likely they were running on propane gas (LPG -
http://en.wikipedia.org/wiki/Autogas), as most of us do. Such conversion can be done virtually on any petrol, or even diesel - weird, I know, vehicle. My Renault is like that and the local dealer converted it for me, but some cars come like that straight from the factory:
http://en.wikipedia.org/wiki/List_of...l_gas_vehicles
Taxi drivers in Sofia love the Volkswagen Touran EcoFuel