Dave Taylor / Davey Herold Question

[RJNorton]

I posted this before, and a few folks offered opinions but nothing specific regarding a source. I think I'll try again. Mike Kauffman has a different version of Harold/Powell at Seward's than any other author I've seen. In American Brutus, Mr. Kauffman writes:

Lewis Powell and David Herold had already gone to Lafayette Park. The Park superintendent always called out the time as he locked the gate, and as soon as that happened, Powell would approach the Sewards' house. He would knock on the door, and then tell the servant he had medicine to give the secretary. It seemed an excellent plan, but at the last minute a complication arose: one of Seward's doctors was still in the house. Now Powell had to either come up with a new cover story or delay his attack. Since nothing came to mind, he sent Harold galloping away to tell the others to hold off.

Herold then rides to the Kirkwood House in an attempt to tell Atzerodt not to do anything yet.

Has anyone ever read this prior to American Brutus? Could the source possibly be something Powell told Eckert? This scenario is not in any other book I've read.