Lex; I looked at all the old slides I have and the one thing I noticed is that, the slides from the 20's & 30's show the switch stands had oil lamps, with no metal targets ( the slide are of the yards around the DRGW system,also the few that I have of remote mainline switches only had oil lamps ), the metal targets didn't seem to show up until the 40's, all the photos in Norwood's books show round metal targets below the oil lamps, and the dates are in the 40's, hope that helps?