TRAIN INDICATOR BOX PT II

MY QUESTIONS OR TERMANOLOGY WAS APPARENTLY INCORRECT ON WHAT A TRAIN INDICATOR BOX WAS ON A NARROW GUAGE ENGINE. THE BOX I AM INQUIRING ABOUT IS MOUNTED ON THE SIDE OF THE BOILER NEAR THE SMOKE BOX ALONG THE FRONT OF THE RUNNING BOARDS ABOVE THE CYLINDERS. THIS IN ALL CASES ON THE D&RGW WAS ON THE FIREMANS SIDE. AN OTTO PERRY PHOTO CLEARLY SHOWS THIS ON PAGE 22 OF THE TRAILS OF THE COLUMBINE SERIES BOOK 1989 EDITION. THIS IS ON A C-21 CLASS LOCOMOTIVE. BOX IS ALSO PRESENT ON PHOTOS OF K-36'S BY 1952. SEE PAGE 151 OF NARROW GUAGE PICTORIAL VOLUME I. WOULD VERY MUCH LIKE TO KNOW WHAT THIS ITEM IS AND ITS FUNCTION.