Reynolds expanded the 531 line-up to include tubesets of varying wall thickness. Some of these tubesets were, from lighest to heaviest: 531P or 531Pro for Professional, 531SL for Special Lightweight, 531C for Competition, 531ST for Special Tourist and 531CS for Club Sport. There may have been more, but this is all that comes to mind at this time. The 531C tubeset was intended for racing or sport touring and to accomodate all weights and levels of riders.

I suspect that what you have is a replacement decal. In 1971 there was only one grade of Reynolds 531 butted tubes. I'm not sure exactly when Reynolds introduced 531C, but I believe it was in the mid-1980s. The Reynolds 531C decal was probably used because was the tubeset that was closest to the 1971 tubeset.