Happy to be corrected here, but it is my understanding that it was common for notices to be sent out to other branches of the forces, asking for young officers to volunteer to serve with the RFC, in view of the great prospective expansion of the branch. Those who volunteered were (at least to begin with) only on secondment, and were not actually expected to leave their own regiments.
A great IWM pic posted last year by Jo ‘Voltigeur’ showing a RIF officer wearing RFC wings (below):
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JT