Thank you for the picture. I agree--there is a lot of stuff in there, and it's hard to access, unless you don't mind lying on your back with the flashlight. Of course, then you are deprived of one hand...
Poirot would be proud of me. I was so frustrated with this that I got out the user manual and went through the illustrated parts list to discover that the mechanism has a SPRING! I found the little guy on the floor near the machine and got to work.
The spring slips over the shaft of the hook and sits on one of the washers. Another washer tops the spring and then the shaft is pushed through the hole from beneath. it's topped by the hooked washer and then the knob is screwed on. When the head is seated the hook snaps under the edge of a metal plate and secures the head. To release it, the knob is pushed to the left, swinging the hook to the right, and one can then tip the head up. This is so cool! I have to admit that I didn't install the spring washers because I didn't have the grip and stability I needed, so my husband (the engineer) is going to do it right. Now I need to find out where to attach the other spring that is hanging from a bracket behind the front panel. It has to do with raising the panel so one's knees will fit under the machine. The panel still moves without it, but it is there for something. Do you have a picture of that?