Richard,
I had my Mistral in the water this morning (pool), and always enjoy it. To maintain it you need a circlip pliers to remove the sintered filter, seat and spring. There is also a pin that extends through the body.
The boxes can be separated using a vice grip and screw driver to remove the clips. With the vice grips, grip the clip, pull them down just a bit, get the screw driver blade behind it, and pry it off with the combined effort of the screw driver and the vice grips. Once out, use the vice grips to flatten the clips a bit to make it much easier to get off (the open end only). Then remove the bottom box (the one with the name place) and the diaphragm.
Examine the diaphragm for holes, cracking, etc. Usually, it will be fine, but could use some conditioning with silicone grease. Coat it, let it sit for a day or two, then remove all the excess.
Examine the duckbill valve; it may need to be replaced. You can get new ones from Vintage Scuba Supply or Vintage Double Hose. These original valves don't last too long; there are new ones of both neoprene and silicone (which lasts much longer).
Remove the two levers, and the pin. There is a small O-ring in the housing which probably should be greased. Put some silicone grease on the pin, then put it down the hole to grease this O-ring.
Reassemble the seat and spring under the sintered filter, and use a pencil with the eraser side, or a flat pencil, to push down the spring and get the circlip into position to re-install with the circlip pliers. You'll need to take the thumb screw out of the yolk to do this.
To set the Mistral for optimum performance, there is a nut over a round thumb wheel just to the side of the platform for the levers. Loosen the nut, and move the thumb wheel to lift or lower the platform just enough to allow the levers to be about 1/4 inch over the top box's opening. Secure the nut, and put the two boxes together, pushing against them very tight (with the diaphragm in place, of course). Put the regulator onto a tank and see whether it leaks. If yes, then you'll need to readjust the levers lower. You want the levers as high as possible without causing a leak. Basically, the platform height adjusts the levers up and down. This is the only thing needed to adjust this regulator. Manuals will tell you to have the lever height at the level of the top box, but I have found that there is better performance with this regulator if the levers are as high as possible without causing a leak.
Now, put the top and bottom boxes together, with the horns oriented symmetrically (look at the label on the bottom box, and put the top box horn in the same relative position as the bottom box. Place the clips onto the box, with the open end down, top (closed end) into the grove of the mating surface. Open your vice grips fairly wide, and clamp these clips down as you go, initially not tight. Two go between the horns, and five below. I always put one just below the horns, then the bottom one, and finally one between the other two on each side. Once they are in place, close your vice grips a quarter turn or so, and re-tighten each one (you'll find that they are pretty loose initially). Do this until they are very tight on the box.
This is a wonderful regulator to play around with, as there simply is not much you can do when you work carefully to hurt it.
John