Yehudi, you may want to hold off on that
if you are talking about the sheave that hangs from a forward tang/tab of the masthead. I have a Kenyon mast w/external spinnaker halyard and the masthead is set up with two tangs/tabs one on either side of the forestay at the mast head. I don't think that these sheaves will work to lead the halyards internally. My main and stay sail halyards run internally and there is one extra halyard exit opening in the lower part of the mast. I can't remember which side. But my guess is that the empty 3rd exist opening is for an extra main halyard from a sheave on the aft/back side of the masthead.Also, I recollect a post on this issue in the last couple of months. Hope this helps.Scott