So, Bob was only a Substitute on the Team?

 

Nora Gregor and Robert Montgomery.

 

Ralph Forbes standing at the trophy table.  That's Neil Hamilton, seated, with the soft hat and the dark glasses.  Behind him is Spencer Tracy.

Guinn Williams, Dr. Wilson, Harry Branders (with broken wrist) and Robert Montgomery helping him off the field.  Bob will change his clothes and take the injured player's place on the field.

At the edge of the field, Bob Montgomery, Henri de la Falaise, Constance Bennett and Guinn Williams.

After the game there was much business of autographing the polo balls.  Below, Bob Montgomery is signing on the stitched line.  Constance Bennett, Mrs. J. Thompson, Henri de la Falaise de la Coudraye and David Manners are also in the group.

And now Constance adds her famous signature.

Constance Talmadge, Mrs. Jack Lawrence, Guinn Williams (back to camera), Johnny Mack Brown and Will Rogers.  

I'd put good money on the possibility that the blurry young man in his polo outfit who is entertaining the ladies, is Mr. Montgomery.