I hadn't heard of the process being used that early, or of the three exposures ("separations") being done side by side on one long plate, but the color-separation process was in common use between the World Wars. One amazing practitioner in Britain was Madame Yevonde:
The technique could be used with an ordinary camera, taking three plates in succession through different filters, but it was faster to use a "one-shot" camera which had one lens, several internal semi-transparent mirrors and filters and three backs for three plateholders, which allowed you to take color pictures onto 3 pieces of black-and-white film or plates with one exposure (as long as you had very powerful lights).
PS: There are apparently no less than 63 prints by Madame Yevonde in the National Portrait Gallery London: