Development of the domestic Kalashnikov variant began in 1959, and the first models submitted by Zastava for military field trials were with the early M64 (or M59) series of rifles with milled receivers, threaded barrels, familiar Zastava handguards, gas cutoffs for grenade launching, and several other diversities from the mainstay AK design, such as a bolt hold open device on the right side of the receiver, and a charging handle that appeared different from other AK models. Though performances were satisfactory, the Yugoslav military did not adopt the rifle as the standard infantry armament.