It is indeed the Boeing Model 15/PW-9. The first Boeing fighter to be operated by the US Army Air Service. It used a 440 hp Curtiss D-12 engine. I'm not sure that the SDASM image was taken on the same day as the background is not the same, and I did not edit the photo I posted. It does not show a registration or any marking on the vertical tail.
longer ron takes the points, but has declared open house.