This is the very first model of pocket-sized scientific calculator produced commercially in the world.
The Hewlett Packard Company started off the design of this device with
the intention of replacing the ubiquitous
slide rule
with an advanced electronic device capable of superior precision
and useability.
This is the result.
The calculator displays ten significant digits and a two digit exponent field (compare that to the three significant digits, and sometimes only two, usually available with a slide rule).
Mr. David Hicks has put together an outstanding
Museum of HP calculators
which you should visit if you're interested in HP devices.
I still use this machine occasionally, and it's bailed me out of more than
one tight spot in its day.