Strax Field Report: The Doctor's Greatest Secret was the fourth Strax Field Report webcast. It was released following the airing of the final episode of the seventh series of Doctor Who and leads into the show's 50th anniversary special. Of the Field Reports released to date, it is the only one to talk about an episode after it had been broadcast.
Field Report D895/O: "The Doctor's greatest secret has been revealed! And it has nothing to do with his chin."
After the events of The Name of the Doctor, Strax reports to Sontar that "the Doctor has kept one of his many faces from view," and shows an image of the new face. He describes this face as having "the look of a battle hardened warrior."
In the designation of this field report, "D895/O", is of particular note. There appear to be some cryptic clues about the War Doctor. In "D89", D likely stands for "Doctor". Put together with the numbers 8 and 9, while not very discernible at the time this episode was released, it apparently indicates this Doctor's placement between the Eighth Doctor and Ninth Doctor, providing one of the earliest hints toward where the War Doctor fit in among the incarnations of the Doctor before any solid proof was given. It wasn't until The Night of the Doctor and The Day of the Doctor that the War Doctor was proven to be an incarnation between the Eighth and Ninth Doctors respectively. "5/O" likely means "50" or the expression "the big five-oh" as in the 50th anniversary of the series.