I have a mission cabinet, but it has the lever. Are there any other pics that would be helpful? They have it pretty closed up in there and it's hard to see around.

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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?

Dont know if these will help. Above are three pieces.
Knob on top.
Then the hook that goes in or out of little hole on machine base. It is like a washer with hook jutting out.
Third is j-type hook that is attached into knob and goes through to the washer/hook above and through base to underside.

Below is the remainder of j-type hook with a nice heavy spring around the pist of this hook.

Note the difference in spring. Tightened is open to lift headd back. Loose is when the abouve washer hook is in hole of base making secure to close lid and drop machine to hide in cabinet.
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Yes, this is what I ended up with, finally. Your pictures are very good. Mine is supposed to have a flanged washer on the top and bottom of the spring, but yours shows a slot the spring's end can tuck into. The washers are slippery and unstable. If your machine is younger than 1909, maybe that's why the design was changed.
Pat

No idea why the uncropped pictures showed up also. I tried to edit out but they show as attachments and I cannot figure out how to delete. Plus they are plus-size! I cropped as did not want to confuse with the material under picture. I have a habit of over oiling and left that under machine a while ago after wiping up and never removed.

Did you notch a groove in it yourself? I love, love, love my machine. The simplicity of it is a marvel to me.