Carter, the thumbs-up gesture was certainly in use in the British Army in 1917. And that may be what Brophy was referring to.

If both thumbs were used accompanied by a verbal "Thumbs-up!" it could fit the description, but in that case I'm surprised he wrote "fingers" instead.

Any other possibilities?

(Contrary to popular belief, the Roman "thumbs-up" was the gesture of disapproval: see OED - which, btw, can't find a reference to the "two fingers" gesture before World War II, or the U.S. "one-finger" gesture before the '50s!)