If you are talking about the thimbles at the top (maybe both ends??) it's to allow the large "D" shackle to pass through the thimbles to attach the shrouds to the mast (the top "D" shackle is larger than a 1/4"- pin size- can't remember right offhand what the size actually is, but the flattened end of the shackle won't go through the smaller thimbles without deforming them). If you MUST use the smaller thimbles you will have to swage the thimbles on to the "D" shackle after you open them slightly to get them on then squeeze them back into shape.
Kirt