The HP-01 (2009)

This was very unusual for 1977, but what makes this watch truly unusual, even today, is that HP integrated the time and date functions into the calculator! Surprisingly, the watch was working and in pretty good physical condition other than a lot of small scratches. The inner ring is holding the plastic battery compartment tight in the housing, the other is holding the two halves of  housing together.

Source: bytecollector.com