Unlike today, during the 1970s Heuer used to specialize in far more than watches - to include stopwatches for sports timing, yachting, science and research (like the example here), automotive, aircraft, and far more.
 
Heuer stopwatches were designed to measure everything from 1/5th a second to 1/100th of a second, and the example here - a 1970s Ref. 508.901 - measures 1/5th a second and was designed for science and research applications, albeit in a very military matte PVD manner.
 
Per Heuer literature of the era, the 1/5th second stopwatches were most frequently used for measurements in laboratories, research, and sport - and particularly humerous (per the same literature), the 508.901 was also recommended for use by...DJs. 
 
The 508.9010 has a distinct military feel to it, in particular its matte PVD black color and rugged black case, with ridged case edges for a more firm grip.
 
Events measured usually were timed events exceeding 10-15 minutes.  Operations are via a central crown, with start, stop, and return to zero by successive depressions of the crown, lending to simple time measurements, for example the duration of chemical reactions, track timing, etc.
 
This Heuer stopwatch comes with a hard plastic travel case.