On 30 November 1967, a powderkeg of dissent within all levels of Bolivian society with its leadership ignited. The multitude of factions within the Army had decided that the policies of General René Barrientos Ortuño were more of a threat than the other factions were, and accordingly moved on La Paz and imprisoned Barrientos in a bloodless coup. After some power-brokering, the left-leaning Colonel Juan José Torres González and General Alfredo Ovando Candía were appointed President and armed forces Chief of Staff respectively. The pair turned to the Eastern Bloc for military and economic aid.
The USSR donated 'new' equipment in the form of secondhand MiGs and tanks, namely MiG-21 interceptors, MiG-17 fighter-bombers, MiG-15UTI trainers and T-54 tanks. All of this kit was taken from the scrapheap and sent in small numbers to replace Bolivia's Second World War equipment.
The Soviet Union provided training and maintenance for the donated equipment, but the eventual downfall of Torres and his government led to the equipment being bartered to the USA in return for American gear.
For anyone with a masochistic streak, the full piece explaining the coup is here