Appears you have one of the original best quality ones, excellent find!
ZKK 600. 601 and 602 series, Fox (Yes center-fires), ZKM 452 series
Every rifle Model with a ZKK, ZKM, ZKW…first initial "Z" is for Zbrojovka Brno. I have a couple in .270 (600) and one rare offering in .223 (601)…love them! However Zbrojovka it self interprets to mean “Factory”. These rifles were designed by Zbrojovka Brno ("BRNO ARMS" stamped) but produced in small quantity 1964-1972 by Zbrojovka Brno. The genuine one's have stamp Z trade mark in circle on top on the receiver (not a CZ) and Zbrojovka Brno N.P on side of the receiver. My personal first ZKK-600 .270 is stamped on left side of the receiver 87. It was CZ made but under Brno license in 1987…Really great quality, almost like Brno, under Brno quality inspector residents still in CZ. This is the last year of Communist subsidized CZ production. 1990 was the beginning of the slide in ZKK quality and accuracy and 1993 was last year ZKK’s were produced. They have since been replaced "new and improved" CZ own 550 series based on... BRNO ZKK DESIGN!!!