Indeed an innovation.
It does sound, though, exactly like the "Call On" that was installed on the Euston-Watford DC Line in the 1930s, apart from the detail that this was a single yellow subsidiary light rather than double flashing white. But in operational procedure, seems pretty much the same. Of course, this was done away with in the 1970s, for a reason which I think nobody knows.
What goes around, comes around.
By extraordinary coincidence this piece of stone-age technology even had special signal aspects for "Flood", as this line had an equal troublespot at Stonebridge Park. I've even found a picture of one on the web here. Perhaps in a generation or two this will make a comeback at Farringdon.
http://www.roscalen.com/signals/London/CWJ.htm#StonebridgePark