Hi, my ensign 1576 has the exact same set up. Not all boats had them , but I like them, I think they are better than the open body turnbuckles , until you have to replace them , then cost will be more. Mine are 50 plus years old and in great shape. Take care not to bend them or get dirt in them. To adjust I use a short crescent wrench so I can do complete circles without hitting the adjacent shroud, and a butter knife or thin flat blade screw driver to keep shroud from spinning. This all you need if they spin freely. If they don't spin freely you need to lube them and carefully free up, a small amount of heat from a torch might help . Once free, put a light amount of silicone grease on threads or tefgel.
To adjust, make sure you start out with both upper shrouds equal length. I put a steel tape on the main halyard and pull it up to with in 6 inches of top of mast. Then measure down to the same spots port and starboard toe rail or winch tops. And get top of mast centered to better than 1/ 8 inch. One method is to measure back equally from bow fitting and make a mark on each toe rail near the shrouds. Then use this mark as your reference. Try a few marks as the boat was not built with perfect symmetry. Easiest with boom off, but if you leave it on make sure it's loose and centered. Leave forward and lowers loose while you do initial equalling off upper shrouds. Get them just firm by hand and an extra two turns tighter from there. Then sight up mast to see if it's straight, adjust as needed. then tighten forward lowers to make a straight mast. To sight a straight mast ,pull main halyard to the slot at the front end of boom. You look at the the spreaders and want to see the halyard centered with the slot of the mast at the spreaders. Leave aft lowers completely loose at this point. Continue tightening uppers and forward lowers a turn at a time, counting turns so everything stays equal length. Probably 5 or six turns from when they became hand tight, but each boat is different. Sail makers Wil recommend 35 to 40 on a loose gauge if you have the right one. Once tight , re check for straightness, and adjust with lower shrouds. When correct you can pull on the stabord upper shrouds and see the top of mast bend to starboard x number of inches. Pull on the port upper with the same force and it should deflect to port exactly the same. This is when you know everything is equal side to side. After all this tighten the aft lowers to just firm by hand then back off a turn or two.
You can pin them in place with new stainless steel cotter pins . If you are happy with your work, get a pair of calipers and measure the the threaded length, write it down and then it's easy to re peat next year. One modification I do is skip the cotter pins and use good electrical tape to keep the adjustment from changing. I am fresh water use only and don't recommend this for salt water as tape leaves the threads wet and causes corrosion.