Stig I was sure it was A8844 but think you're right about A6644; 644-646 were N17 purchases from France and 6644 was shot down with 1 Sqn. 18 May 1917 ( 2/Lt. T.H. Lines POW).
Easy confirmation when a close-up though as here....
https://commons.wikimedia.org/wiki/F...lane_A6686.jpg
Nick