I've got an 87 Pearson 28, which I believe was very much like the 27. Like Jack, I've got single line reefing, which is the way the boat came. Of course someone could have easily changed yours over the years.
The single line works great for us now that we put new 3/8" lines in and cleaned up the bullet blocks that run inside the boom. Friction is the enemy and old fuzzy lines or really large lines increase friction.
Each reefing line is actually two lines in the Isomat boom.
* The line for the leech of the sail goes under a sheave at the end of the boom, around the end sheave of the bullet block and back to the inside end of the boom where it is tied off. You'll want the working end of the line to go through the cringle and back down to boom where it is secured in some way. If you don't do this the leech will be pulled back but not down.
* The line for the luff side starts back in the cockpit, makes its way through deck organizers and blocks to go over a sheave at the mast end of the boom, around the sheave at the forward end of the bullet block, and back out the forward end of the boom and up to the cringle.
Someone else can chime in on two-line reefing--more lines but generally less friction.
