Hi, Richard:
I can answer your question only by saying that in my time with Doug Schnarbush, he told me the flanger was operated by a valve on the engineer's side of the cab. Having never operated it himself as a fireman, he could not be more specific.
This has been a troublesome question for me as well, because I have never seen any evidence of a second air hose on the back of any C&S tender. And making things more frustrating, pictures of Flanger 015 don't even show evidence of a second hose on the front. I have wondered if the lead engine operated the flanger blade through the trainline air hose, but it seems like this would be in violation of every rule known to man, not to mention common sense.
There is an air cylinder visible in pictures of the 015, so it was air operated for sure.
Let's see if we can find pictures of the wreck in '36 of the 75 and 73. They had a "car flanger" coupled between them. Maybe there will be a clue or two to be found there.
Mike