You mean when did they stop putting a braze on for the pulley wheel? My impression is that it was only the high-end IGH that had the braze on (my '52 Lenton Sports and '50 Lenton Tourist have one; I've had other Raleighs from that era that had the clamp-on wheel guide). Those higher-end IGH bikes seemed to be gone by 1957 or so when the Lenton came with derailleurs.