And the photo, in case anyone is still wondering, was to accompany I42's original reference to the AW-19.
The "A3" is from the 1929-1947 B-class test registration sequence, where an individual letter prefix was allocated to each UK aircraft manufacturer for test flying purposes. Armstrong Whitworth got "A", beating Avro (who got "K").
Full list of prefixes here:
https://en.wikipedia.org/wiki/United...t_test_serials