As far as I know, there are three possible variants.
Earlier version with one FIXED Lewis on top of wing. Second one replaces with one synchronized Vickers over nose.
Third one, RFC version, with Vickers and FLEXIBLE Lewis on top. Check this table:
http://pease1.unh.edu/bluemax/aircraft/cchart.gif
(note that Youplay chart incorporates Medium Burst value)
This additionally gun must suffer Flexible Gunnery penalty, so it will never be equal to 2-gun plane. Imagine yourself aiming plane's nose mg AND aiming second gun up and down with hand. Wierd.

And no, Nieuport 17 in early war not flawed, I usually outrun Albatros DIII (sometimes two). Very well balanced