Unlike today, 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. Its stopwatches were designed to measure everything from 1/5th a second to 1/100th of a second, and the example here - a Ref. 758.901 - measures 1/5th a second and was designed for science and research applications.
Per Heuer literature of the era, the 1/5th second stopwatches were most frequently used for measurements in laboratories, research, and sport. But this one here has a rather distinctly military feel to it, in particular its 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.
1983 Heuer Ref. 758.901 1/5 Sec Military Stopwatch
DIAL: Original Heuer-signed white dial, with original minute and second hands.
CASE: Original black 62mm x 84.5mm rugged plastic case. Original black Heuer caseback.
CRYSTAL: Original crystal - no scratches.
MOVEMENT: Original "shockprotected" seven-jewel Heuer manual mechanical movement.
CROWN: Original unsigned crown; original minute and second hand reset pusher.