Check your sail to see where it...
...may be missing from. Could either be a sail slug from the luff (forward edge of sail against the mast) where there are many or from the clew (aft most portion of the foot of the sail, part that goes against the boom) where there may only be one. I can't tell from the picture and as other's pointed out it may have been a replacement, but does look like the ones at the clew. I have Isomast Spars, what type are yours? One other thing, it may be a "long lost" slug that was left where-ever and already replaced. alan