Charles "Slim" Lindbergh (l) and Harlan "Bud" Gurney (r) standing by a Robertson DH-4 mail plane.  Exact date
unknown.  Notice the landing lights on the wings and the lengthened exhaust pipe to aid the pilot's night vision.
(photo: "The Spirit of St. Louis" by Charles A. Lindbergh)

Some observations about this photograph...
This is an interesting picture to try to date.  The airplane in the photo does not look like one of the original four DH-4s (#s 109, 110, 111, 112) that CAM-2 started out with in April 1926.  Notice the landing lights installed on the top of the bottom wing, just outside of the outer wing struts.  Also, notice the lengthened exhaust stacks and darkened windscreen on this later Robertson DH-4.  The presence of a government issued registration number on the tail (see listing at Aerofiles web site) would date the picture as after March 1927, when numbers were first issued.  Additionally, judging by the fact that there are eleven Ryan B-1 Broughams in the registration list ahead of this DH-4, and that Ryan B-1s did not leave the Ryan factory until the summer of 1927, and that there is snow on the ground in the picture, I would say this photo was probably taken in late 1927 or early 1928.