,,Anton uses two cells (3V) for the HV power supply generator, and three more cells (4.5V) for the pulse and audio circuit. Almost as if the two circuits were designed separately and then spliced together. Also note Anton uses a transformer coupled output to the headphone. ( per my Model 5 manual). Simple, straightforward.
In the Model 6, the Victoreen folks took a little different approach. They stacked 4 cells one on top of the other ( electrically) and took a tap at the 3 volt level. If you were to redraw the circuit, it would look like the HV generator runs on one 3 volts and the audio /pulse circuit on another 3 Volts. Only in one place is the total 6 V used, and to my eyes, it could have been done better differently. Resistive headphone coupling which also leaves a little DC on there, but it's held at a low level by a diode clipper. 
Lionel ( Model 6B) comes along and really shakes things up. Two cells only, because you only NEED two cells. Both the HV and audio run on the same 3 Volts. Capacitive headphone coupling eliminates a costly transformer, and does away with any DC component at the same time. Only 3 transistors instead of 4 also.
E-N has four cells, but they are series parallel. Only 3 volts, but twice the life? Curious because parallel dry cells tend to discharge one another. Three transistors in a very similar circuit to the Lionel, with a major exepton that there is no HV regulator tube. Instead a Zener in the primary provides regulation. 
All the above use a standard approach to coupling the pulse from the GM tube. HV is applies to the tube via a high values resistor, and the pulse is coupled via a series capacitor, which also blocks the DC from getting into the audio stages.
In one version, the 6A, Victoreen took a real leap. Still 4 batteries, but now only 2 transistors. A very complex transformer couples the pulse directly from the GM tube top the single stage audio amp, and another winding drives the meter directly. Sheesh.
And there may well be other variations. Seems the individual manufacturers were given a free hand in the actual design, rather than built to a specification. Also the individual companies made the discreet parts involved, in many instances. Especially GM tubes, and transformers. Something can be said for diversification I suppose. Perhaps it was simply that so many had to be delivered in such a short time."

The only CD V-700 with a plastic case was the Universal Atomics CD V-700 model 4 (picture to the left), which has also the poorest quality.

Check source from the Lionel CD V-700 6b

Every CD V-700 has a check source on the side. This should have about 2.5 mR/hr with beta-shield open, but some can have even 1 mR/hr (if they use Radium D+E, which has half-life of 22 years). They used Radium D+E or depleted Uranium as a check source. 

There were also made modifications on the original CD V-700 as for example the CD V-700M, this modification allowed to detect Alpha radiation, click here to read more about this.

Another modification, the CD V-700RP but here was changed only the probe (no HV modification). I am not sure if it is an official Civil Defense modification... Here is more

"What you have is a CD V-700 that was intended for use with one of the B-2 aerial monitoring kits. The kits originally came with an OCD ITEM NO. OCD-D-101, TYPE A Geiger-Mueller Tube and a "stick-on" range change. The modification G-M tube and applique was manufactured by EON Corporation. That G-M tube was designed to be compatable with the CD V-700. Changing the GM tube effectively decreased the sensitivity of the CD V-700 by a factor of 10 due to the decreased volume of the G-M detector. Thus the ranges became 0-5, 0-50 and 0-500 mR/hr."

The new range