Hi, I hope you had a fine day today!
Tonight I did some more testing with the Spotmatic. After some 20...30 shots, the camera misfired as usual. I immediately opened the bottom plate of the camera and took a photo of the positions of the mechanisms. The "spring lever" that
pentaxus mentioned is in its normal (uncocked) position after a misfire. The lever doesn't seem to get jammed like in that Spotmatic F Youtube video.
Then I just re-cocked the camera and the shutter tripped happily again like nobody's business.
Here's two pics that show the mechanisms. Sorry for the second pic, I missed the focus a bit on that one.
Misfiring Pentax Spotmatic - Album on Imgur