Late,I know,but it looks to me that the "sign" is a printing error.The shadow on the jacket appears to be continuous across the boundary.In the opening post does it not say "....it does not appear in the original...."?
That's an interesting observation. I'd taken the phrase "....it does not appear in the original...." to mean it was equally illegible there, so maybe Mhillbilly can clarify that.
Meanwhile, you've inspired me to investigate further....
The image in my browser window (shrink to fit, and excluding the white margins) is about 227mm x 157mm. This is almost exactly the same proportion as a roll film negative image of 3.25" x 2.25" (approx 1.44:1); according to Wikipedia, this format was introduced in 1897. Other negative and plate sizes have different proportions - see
https://en.wikipedia.org/wiki/Film_format#Still_photography_film_formatsSo, if the white 'sign' was not just a printing error, but an unexposed part of the paper caused by a piece of paper stuck on the negative, I calculate that the paper would have been about 0.84" x 0.27". And the two darker spots on the right of it - could they be holes made by a staple, or stitching? They seem to be about 0.31" apart.
Could this mean anything, or is it a bit fanciful? And why might it have happened? Was it a label that had been stuck on to identify the roll of film, but they had to use that image because it was the only one? Did the chap have his flies undone? Had a pigeon pooped on him?
As so often, more questions than answers...