Source : Free On-Line Dictionary of Computing

Seiko RC-4000
     
        A wristwatch with an {EIA-232} interface.  A clip fitted round
        the watch and made electrical contact.  This clip had a socket
        for a stereo style jack lead the other end of which was a
        25-way {D-type} connector.  The lead allowed you to enter
        phone numbers etc. into the watch without having to play with
        tiny buttons.  It also meant if the battery on your watch ran
        out you could restore the data without having to type it all
        in again.
     
        It was around the era of the 8-bit home computers like the
        {Spectrum}, {BBC Microcomputer}, {Apple II}, {C64} - the
        1980s.
     
        (1995-01-30)