I opened my new FED to find out how it worked. The bottom cover was easy to remove, having all screws apparent. However, it hid nothing of interest, only the battery case.
The top cover was another affair, indeed. I first thought that the levers were ‘clipped’ in place, but no. The film advance lever can be removed by unscrewing the top part of it (use a large pair of pliers, protected with tape or cloth so you won't scratch the paint if you loose grip). Then it's easy to get the lever and washers off. There is a last threaded washer to unscrew on the axis of that lever.
The ASA setting knob unmounts the same way.
The rewinder is easier to unmount; open the back of the camera, hold the axis with a plier and the black top of the winder unscrews from the axis. (Pay attention when rescrewing all these, it's easy to force the threadings.) There's a last small screw to remove, under the rewinder. You can now lift the top cover, but be careful of the flash cable.
The exposure mechanism of the FED is astoundingly simple. There's a Light Sensitive Resistor with a little plate in front of it. The plate has several holes in it. The diameter of the hole determines how much light is allowed in, and this is the ASA setting wheel. (If you remove this wheel, be careful that on the up left of the LSR there's a small spring with a steel ball that ‘clicks’ the wheel in position when you select an ASA setting.)
The LSR is connected to the battery and to an old fashioned VU-meter. The needle of the VU-meter moves accordingly to the quantity of light that comes on the LSR. When you're in ‘A’ mode, and depress the knob, a cam comes on the needle, and select a f-stop accordingly to the needle's position. I could not get further, however because I did not see how to split the camera in two, so I could see the shutter mechanism at work. (I always loved that kind of clock- like mechanisms ;-)