WW1 RAF night bomber markings question?

I came across an interesting reference in a history of aircraft markings
book to an apparently short lived, early war upper-wing national marking for
WW1 RAF night bombers. The reference described it simply as a white disc
used only on night bombers. This marking was different from the white discs
used on fuselage sides as a squadron marking.
Can anyone provide any other information regarding this marking?
Was it simply a white disc? What year was it indroduced? Withdrawn?

Thanks for the help,

Wm. (Bill) Burns
London, Ont.
http://www3.sympatico.ca/wmburns/

"Those who forget the past are condemned to repeat it"