According to extant catalogues, the matched set 881/881B Tour de France rear dropouts debuted circa 1962, the same time as with the Prestige derailleurs. Prior to that, the TdF dropout set used was an 880/881 combination with the drive side dropout having an integral hanger. The switch from 880/881 to 880/880B also saw the name on the dropouts changed from LJ SIMPLEX to JUY SIMPLEX. These dropouts were discontinued circa 1978, so they had a long production run. However, given the other frame attributes, I'd place this in the 1960s end of the time frame.