Until Faster pointed that out earlier in this highjacked thread, I never realized what they were used for. The PO had both jib halyards outside of these wrap preventers (one halyard's actually intended for use with a spinnaker or staysail).
Consequently, when the masts were unstepped in '04 after our purchase of the boat, we didn't thread them through. It seems as though the U-brackets need to be unscrewed to do so, since the shackles will certainly not fit.
True Blue . . .
sold the Nauticat