The mid-production (I'm not sure about early and late production) Mariners did come with a roller-reefing arrangement where the boom was rotated to reduce the size of the mainsail. The gooseneck permitted (still does on most Mariners) boom rolling by pushing the gooseneck out slightly against a spring so that a round pin was in the boom, and not the square pin. There was also a large boom "claw" to which to attach the main sheet (most Mariners have lost that piece a long time ago). The claw was slid over the boom end after reefing the sail, and then the main sheet attached.
However, the system sucked (and still sucks). You get a terrible reefed sail shape. It takes way too long to accomplish the reef. You lose the ability to use a boom vang - which is really critical on the Mariner to prevent the boom from rising and catching on the backstay during a gybe.
Do your self a big favor, and have a sailmaker install reef points (2 sets if you sail in a windy area like I do) on your mainsail. Then arrange for a hook or use the downhaul to pull the tack down, and a clew reef line to to position the reefing clew on the boom.
The Mariner has a very large mainsail for the size of boat. If you don't have rail meat readily available, you are going to want to reef the main at about 15kts of wind. My Mariner is now set up to allow me to reef underway by heaving to, putting in the reef, and then going on my way. Clew and tack reef lines are pre-rigged when the main is bent on.
The Mariner Class Association (
https://usmariner.org/) is a great resource for all things Mariner. Well worth the $15 per year.
Fred W
Stuart (ODay) Mariner #4133 Sweet P