From the display it appears to be an alarm model that hasn't been set, since the 1 oclock position is not working. I'm under the impression that the alarms are more common, but that could just be a factor of people who have a stopwatch model don't part with them as often. The alarm is easy to use and set once you understand it, but it is not completely intuitive, so again, that could skew any market perception.
The cases are not interchagable, as the alarm back is held in by a ring, while the stopwatch has a one piece screw back. Different diameter threads

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