The Model 12 is a high sensitivity gamma μR/meter employing an internally housed 1"X1" thick NaI detector. The aluminum cast instrument housing with its separate battery compartment and accompanying metal make this a rugged meter.
The first scale is 0-3 uR/hr which is actually less than a typical background, so you know it can see even minor variations in background radiation. Much more sensitive than a Model 19 uR meter to which it is comparable
The Model 12S is a complete radiation measuring device with integral detector. It is everything you need to start measuring radiation.