Radio Shack TRS-80 PC-2.
All of Tandy's calculators and pocket computers were OEMed from either Sharp or Casio, and this unit is no exception - it is in fact a Sharp PC-1500 and can exchange accessories with the PC-1500 and PC-1600 series computers.
The photograph makes this unit look deceptively small; it runs off 4 'AA' cells and is actually quite thick and heavy. (Contrast this with the Sharp PC-1403, which runs off two CR2032 lithium cells and is truly a "pocket computer"). The PC-2 is about the same size as the Atari Portfolio, but much less powerful.
The PC-2 is a pure computer, lacking the general-purpose calculator keyboard of many other Sharp models. All the same functionality is, however, available by typing in the appropriate keywords. Because of the additional typing burden, this device is probably not so useful for the engineer's shirt pocket. I find my PC-2 useful for functions such as working out component values from their codes and vice versa; tasks that I can do in my head, but that are better performed by a program in order to avoid stupid mistakes.
For a wealth of useful information and downloadable manuals, software etc., visit the Sharp PC-1500 web site.