Those look like part of a system you sometimes see used for stowing a mainsail. A length of bungee cord is left running through the "Cs." When you want to stow the mainsail, you just pull the bungee cord over the sail and hook it on the "Js." Not something I would use with fancy sails like yours and not really needed on an M17-size mainsail in any event. -----Original Message----- From: montgomery_boats <montgomery_boats-bounces@mailman.xmission.com> On Behalf Of Jason Leckie Sent: Monday, August 5, 2019 1:50 PM To: For and about Montgomery Sailboats <montgomery_boats@mailman.xmission.com> Subject: Re: M_Boats: Reefing the Mainsail Hi John, Here are a couple of pictures I have pulled out of my albums. The first picture, if you can zoom in, is of the 'C' and the second is of the 'J' hook. I have been tying a reef knot through that strap eye, which you called it and around the boom, so maybe that J shaped piece is almost like a cleat? Thanks, Jason.