Dear Higgs,
From the picture, it appears to be an "A Frame" mainsheet--operated from one side, with the sheet beginning at a becket on a standing block (either port or starboard, whichever is the operating side), running freely through the first sheave of a double block on the boom, down to the opposite side and through a second standing block, back through the second sheave on the boom and finally through the first standing block sheave to a winch/cleat arrangement.
I have this exact (as described above) setup on my 32' O'Day CC, except that I have end-boom, rather than mid-boom. While the set-up is original O'Day installation, circa 1976, I am going to change out for a aft cabintop 60" traveler, because I don't like the way the sheet, when released to de-power the sail, changes the sail shape as well as changing the angle of attack.
The vast majority of my 40 accumulated year of sailing experience are with traveler rigs and I've become very accustomed to using the traveler to de-power the main when necessary (by releasing) while NOT changing the sail shape. The A-frame set-up does control the sail and, in the conditions I've sailed it to date, is OK, but it's not what I've become accustomed to and since I can make the change without an inordinate expenditure (craigslist & Ebay are great, if used judicially), I'm going where my experience leads me.
Depending upon your sailing personal sailing preferences (See thread 'Outhaul'), the rig might be just what you need/want and it might be great for your situation.
Fair winds & following seas!
