Most if not all model years until 1985 of the Day Sailer had a roller-reefing setup built into the boom gooseneck, as mentioned above you pull the boom aft (while attached to the mast) this disengages the square part of the gooseneck from the recess in the forward end of the boom and you can rotate the boom to roll part of the sail around it (I forgot to mention the need to disconnect the mainsheet from the mid-boom attachment point) ease the halyard and roll up the sail to the lowest batten or less. Release the boom and it SHOULD slide back over that square part of the gooseneck preventing it from unrolling. Then reattach the mainsheet using the optional reefing claw as the sail is now covering the mid-boom attachment point. Retighten the halyard and you are reefed. My experience is that it might work that well, but more often than not the boltrope on the luff of the sail causes the sail to roll up more along the luff than the leech resulting in the boom sagging more aft, and frequently the boltrope also prevents the gooseneck from relocking so the sail often starts to unroll after a while. Biggest problem is that with the mainsail roller-reefed, you can not longer use a boom vang (Just when you REALLY need one to flatten the sail!). I tried using the standard roller-reefing setup for the first couple of years that I had my boat, then had a local sailmaker add reefpoints to my sail allowing me to reef the sail with a so-called "jiffy-reefing" or slab-reefing setup, the DS III and Cape Cod Shipbuilding-built DS IV have this setup as standard. WOW, what a difference! The sail sets much better while reefed this way, I don't have to mess with the mainsheet attachment (sold my reefing claw to help offset the cost of the reefpoints) and I can still use my boom-vang while reefed. My advice would be to add reef-points to the sail (cost is under $100 total) and forget about the roller-reefing. We did the same upgrade on our old CAL 21, and never regretted it. Properly set up, roller-reefing can work, but it is at best a make-do set up (despite the sales pitch made by so many boat-builders in the late 1960's and early 1970's about it being the best thing since sliced bread!) Roller-reefing results in a sail that can not be flattened to depower it and reduce heeling forces in strong winds, slab-reefing results in a much better sail set.
One of the features that attracted me to the DS II was the standard roller-reefing (especially when I discovered that the original owner must have purchased the optional reefing claw needed!), but I never got the reefing to work right due to the problems caused by the boltrope.
However, for reference, here is the section of the DS II owners Manual describing how to reef.
Roller Reefing: A “Roller Reefing Claw” is necessary. (See Dealer)
Your Mainsail can be easily reefed, as the boat is equipped with a spring-loaded gooseneck. First, remove the block in the middle of the boom (detach the Boom-Vang pendant from the boom, if so equipped). Second, release the main halyard, but keep it under tension. Third, pull the boom back from the mast so that you can turn it. Fourth, roll the boom either way as you or your crew lets off slowly on the halyard. The sail will roll on the boom. Fifth, when you have rolled about 5-6 times, you will have reduced your sail area by 1/3. Experience will teach you how much to reef under various conditions. Sixth, lock your boom back in place by letting the boom go forward and tighten up the halyard. When reefed, the boom block for the mainsheet is attached to the roller-reefing claw. To shake out the reef, just reverse procedure.
And a picture of the reefing claw, might be possible to make one as they are pretty rare these days and not cheap.