Okay, I opened the top up to find that the blade and slots engage properly, but the whole shutter speed shaft moves when I nudge it CCW to have it snap in place. (Following is bad terminology, I apologize, but I've never opened a camera before.)
Looking at the bottom, I found what the issue is, though I'm not sure what causes it and what to tweak to fix it. The following pictures are on the bottom of the camera (front facing right), directly below the speed knob and curtain gears, under a small plate held by two screws.
The tabbed disc, held to the shaft of one of the curtains by a screw, in the center of the image, rotates CW (as shown) as you cock the shutter, and is supposed to pass under the curved tip of the metal spring and hook onto it. Unfortunately it doesn't make it there.
Here the shutter is cocked, and you can see the tab stuck right under the tip of the spring. Now, if I turn the speed knob a little, CCW looking from the top, the tab completes its motion and snaps in place.
Like so. I can make it snap properly while winding but I have to push seemingly unreasonably hard on the lever. As in keep pushing after it stops moving.
I see two obvious ways of solving this, one is readjusting the position of the tab (as long as it's not keyed), but the screw is tight and the whole thing flexes a little (did not try to loosen it, only checked if it was tight).
The second is readjusting the position of the spring, which seems more sensible since its screw is glued down, which means it was and can be calibrated. I'm not confident I can do a good job though.
There might also be some other problem upstream that produces this behavior. The curtain gears look clean, the camera seems used very little.
What do you recommend? Thanks a lot.