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The advice given on this site is based upon individual or quoted experience, yours may differ.
The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.
89 catalina 25, my main sail halyards exit from inside the mast on the starboard, jib on port and then there is a third set of lines going to the masthead to a pulley that are external and on the stern side of mast...what are those for?
I agree, sounds like a topping lift. In light air, a topping lift prevents the weight of the boom from affecting sail shape. At anchor, it allows you to swing a small person over the side of the boat and torment them by slowly dipping them toward the water (I've read).
A topping lift also supports the boom for lowering or reefing the sail without having to clip it to the backstay (I never liked to clip the boom to that pigtail when the wind was getting so strong that I wanted to lower or reduce the main). If your mystery halyard has some kind of clip on one end, that can clip into one of the holes in the fin on top of the boom-end casting, and if it's long enough for the other end to reach a cleat on or near the mast (you didn't indicate that your halyards are run back to the cockpit), then you can use it as a topping lift. I know that a lot of people don't have one, and probably don't feel the need for one with a mainsail as small as we use, but it's very handy once you get used to having it.
Notice: The advice given on this site is based upon individual or quoted experience, yours may differ. The Officers, Staff and members of this site only provide information based upon the concept that anyone utilizing this information does so at their own risk and holds harmless all contributors to this site.