Most LeTours were Panasonic built, so the serial system is entirely different. Show us a pic, but looking at your serial, I'm guessing it's actually 5D934028. If that's the case, it's likely from April 1975 or April 1985 (this is why pics would help).... but it also could be that your bike is neither Schwinn nor Panasonic built.... so, yeah.....pictures.