A potential for this new aircraft was distinguished very soon and therefore in 1973 developement an armed version was started, especially because of the export ability to the countries of the "Third World". This modification was able to be used for advanced weapons training by placing an extra hardpoint under each wing - therefore increasing the number of hardpoints to four. The modification with four hardpoints on the reinforced wing and with ability to carry two external droptanks on inboard pylons, got airborne for the first time on 25 June 1975 as the ninth prototype was designated the L-39ZO. A year later the ninth and tenth prototypes concluded combined contractual and verification tests.