We have been discussing Stewart Plectrum Banjo in separate thread. http://www.banjohangout.org/topic/229305/2/#2912663
This thread inquires about the history of the Plectrum Banjo in general.
When and by whom was the Plectrum Banjo concieved?
Why?
Who was the first manufacturer to make a Plectrum Banjo?
Here is an interesting June 13, 1914 advertisement by Walter Jacobs publishing house. The ad offers musical arrangements for the Plectrum Banjo as part of the Mandolin Orchestra. http://mtr.arcade-museum.com/MTR-1914-58-24/index.php?page_no=147&fname=MTR-1914-58-24-147.pdf
Do you suppose that the 4-string Plectrum Banjo was concieved to supply a plectral banjo voice in the Mandolin Orchestra and that was its original purpose? Of course, the 5-string banjo had been a part of Mandolin Orchestras prior to 1914. But the finger style technique for playing the 5-string had at least one major drawback, namely, the difficulty in sustaining notes. Very few classic finger style 5-string banjoists could tremolo successfully. A 4-string banjo built upon the regular (regulation) banjo model and played with a pick could be made to render a desireable tremolo and sustain the notes required by the musical piece chosen for the Mandonlin orchestra.