Need help deciphering photo heading from B17 crew picture

My father was on a B17 and shot down over Germany on his second mission. He was a POW for 18 months. I have a picture of him with his crew. In the upper left hand corner of the picture is the following:
(518AB 3 G 42 470) CREW 26 REEVES GROUP
I am trying to figure out his unit information and potentially what aircraft name and ser#.
Any help would be greatly appreciated.