I have no idea but a quick consult with the oracle at Google shows some interesting tidbits....
from Treadleon
"This is a very interesting pattern, sometimes called the "floating bird", also "RAF". It seems to be most commonly found on Model 15-96, in Canada and England. The bed picture is of a floating bird, usually considered an eagle. Note the lightning bolts on the arm. This pattern seems to date from just before and after WW II."
From ISMACS"
"RAF" (colloq.), Model 15, c.1935 to c.1950s
From
http://www.singersewinginfo.co.uk/15/
"During the 1930’s 15K models built in the Kilbowie plant in Scotland were commonly finished with ‘RAF’ decals. These were originally designed by William Hopper of Dumbarton in Scotland who worked at the Singer plant at Kilbowie."