Although this is not an actual prop from the movie, this Seiko stopwatch is the same model that Doc Brown uses to prove that Einstein has just made history by traveling one minute into the future!

The silver bag contains a printer that is connected to the stopwatch.  As you can see, this stopwatch has an LCD display.  The close-up shot shows an LED display.  The was most probably done to ensure that it was readable during this key scene as an LCD display would be hard to see at night.  If you watch closely in the scenes leading up to the close-up, you can see enough of the watch face to see that it does not have an LED display.  The close-up was shot with two different stopwatches that had been modified.  The two were connected by wires to a control box that had controls for setting any time.  For this scene of course, it was set for 1:21.  I've actually seen this prop and it still works just as in the movie.  This is a very good example of the attention to detail that went into the props for this movie.