Neil Wiseman kindly sent me some pictures of the wooden strut mentioned here, one picture is dated 1896 and the other from before 1901. Stone 56 is leaning and lintel 122 can just be seen peeping out from behind stone 16.
As always I turned to my trusty Gowland - his report on his straightening of Stone 56. And Fig 1 shows no strut.
So when was the strut removed and why?
It looks to me as though stone 7 has been straightened a little. And if you were so afraid it was going to fall that a large wooden strut was constructed I think some remedial work would have been done before removing it. Later it had larch poles erected to help support it and then it was lifted and concreted into the ground upright in 1919-1920.
UPDATE: Analysis of the photos by Simon Banton and others leads them, and me, to the conclusion that the stone wasn't straightened, it is an effect of the different camera angles.
UPDATE 2 : From Hugo Anderson-Whymark: "I was interested in this structure and had a few e-mails with Martyn Barber last year. He told me that it was erected by JJ Cole, an architect, to ensure the stones didn't collapse before straightening (which didn't happen at the time) and the structure was removed during Judd's 1901 works. Apparently a visitor in 1881 described it as a 'gallows-like erection', as your photos clearly show.
J J Cole was Antrobus's architect according to Rosemary Hill's Stonehenge and it was his suggestion that the monument was made safe rather than restored, and that work done was obvious. A very modern and enlightened view.