Original CZ was a CZ-6; released in the Fall of 1991. Proto's had been running around since Fall of 1990.... and had been on the Dave Johnson drawing board and test bench since 1986. ((( Dave never felt he truly 'completed' the project.... as his time & funds were FRL restricted ))). Then; CZ-5 was released in 1992 (slightly lower cost unit for more folks to afford).
CZ-7 = Fall, 1996
CZ-7a = Spring, 1998
CZ-7a Pro = Summer, 1998
CZ-70 Pro = Sept. 2001
CZ-3D = Feb. 2004
The CZ-6 was; analog, rainproof/splashproof/dustproof... no external speaker.
The CZ-5 was; analog, NOT rain/splash/dustproof.... had external speaker.
The 7, 7a, 7a Pro & 70 were digital and rainproof/splashproof/dustproof.
The 7, 7a had "AA" batteries under arm-cup. (7a corrected a battery drain problem).
7a Pro & 70 used a pair of 9V in control box.
CZ-3D was; analog, NOT rain/splash/dustproof.... has external speaker.
Depth capabilities have been nearly identical since inception; with a couple of minor exceptions = CZ-70's were set-up so as to authorize a little bit of 'overdrive' in the Sensitivity dept. When Sens was set at '6' or above, the detector might become slightly chattery/unstable. Some areas could handle this; hence, this performance availability. The CZ-3D was also set up in this fashion; but, in addition, it will ID deeper coins (and other targets) with greater ID accuracy. I chose the NON-rainproof/dustproof/splashproof box config for the CZ-3D so as to allow more folks to afford the unit...... and analog platform as most prefer this config. --The CZ-20 is a analog CZ-5 in a 250' deep capable underwater unit.
Interesting that no unit has been able to trump the depth capabilities of the CZ platform in most applications.