My Journey on Lincoln's Assassination

[Warren]

"There were plenty of returning soldiers, and many of them were on crutches, unattached, and the same size as Booth. James Boyd also had the bad luck to have a wide scar on the back of his neck. When Major O'Beirne had all the pieces in place, he telegraphed this fact to Edwin Stanton and also notified Booth and Herold." (claim of Dr. Arnold)

Just curious, but at what point in time did Major O'Beirne know that he would need a doppelganger that required a crutch? No one except Davy knew the extent of Booth's injury until he was treated by Dr. Mudd, and that is the first time he used a crutch. The bridge guard would not have known. When did O'Beirne telegraph Stanton that he had found the perfect double with convenient leg injury and scar, and how and when could he have notified Herold and Booth of their good luck?