Staff officer's armband or 'brassard' measuring four-and-one-half by eighteen-and-one-half inches. This is the second version of the staff officers armband introduced in February, 1916.  The first version introduced 1915 did not carry the initials RFC. The lower left photo is taken from a larger photo of the A and C flights of No. 57 Squadron.  This observer apparently is wearing the same armband on his right arm, possibly to indicate an officer-of-the-day assignment.  In the center photograph, a flying officer with the RAF in Canada wears a Provost Marshal's armband of similar design but different initials.  The wearer in the right photo is Padre Keymer of No. 40 Squadron in the spring of 1918. 

The drawings below are from Andrew & Peter Cormack's Osprey Men-at-Arms Series British Air Forces 1914-1918 (1) and (2) which was researched from the collections of the RAF Museum Hendon.  The photograph in the bottom right corner is from that collection.  The officer wearing the RFC armband is Barfoot-Saunt.