The CD V-700 also came with a "check source", a bit of a radioactive isotope under a sticker on the side of the unit. The isotope varied with the maker; depleted or natural uranium was common, though the Instruction and Maintenance Manual for the Lionel Model 6B indicates that a "Radium D+E beta source" with an approximate half-life of 22 years is present under the nameplate. This produced, at the time of installation on the machine, about 1–2 mR/hr adjacent to the source, a value which was clearly visible on the analog meter as well as audible via the headphones that accompanied the units.  Measured with the probes beta window uncovered and the probe directly in contact with the operational check source, this is about 100 times normal sea-level background levels of radiation, and similar to the near-field from a uranium oxide-glazed red Fiestaware saucer.  The level drops back to background levels a few feet from the source.  Since the half-life of the different operational check source materials used varied and since all CD V-700s were manufactured prior to 1964 and are now approaching a half-century old, the amount of  radiation emitted from the source may be much lower than when the unit was originally issued.  Therefore, accurate calibration via the source cannot be assured.
The units were calibrated at the factory, but this may drift over time and need to be recalibrated by means of an adjusting screw inside the case. The procedure for this will vary from model to model and it is best performed by somebody that is familiar with radiation metrology gear and its calibration. US state emergency management agencies usually maintain a calibration lab intended to keep the state's own inventory of such devices calibrated and often offer such services to local first responding agencies statewide but generally do not offer them to the public.