This picture shows the family of CD V720's. To the far right is the Victoreen Fallout Detection Meter a commercially sold version of the 720 without the opening bottom.
More on the Victoreen FDM here. At the bottom of the picture is a Victoreen 3A open.

     The CD V-720 is one of the early issue Civil Defense instruments. I haven't been able to find an exact issue date for the first CD V-720s. The first 720's were originally issued in the late 1950's with the CD V-710. The Victoreen Model-2 is the only CD V-720 that I know of that was manufactured with a plastic case. The early versions of the CD V-720 (Chatham and Victoreen Model-2) were declared obsolete and went through disposition like the entire family of CD V-710s. Many of the early CD V-720s that are found today have had the labels defaced and the "CD" logos marked over. This was done during the disposition process.

     The CD V-720 is a high-range radiological survey meter that can discriminate between Beta and Gamma Radiation but only measures Gamma. The range of the CD V-720 is 0-500 Roentgens per hour. The sliding cover on the bottom of the meter case blocks Beta particles when closed allowing only Gamma radiation to pass through the detector. When the cover is opened this allows Beta particles to pass through the very thin metal membrane on the bottom of the CD V-720's Ion Chamber resulting in detection of beta radiation. See picture below.