I think I made a discovery that potentially explains my troubles. Ok see on this web image
http://4.bp.blogspot.com/-BK4-xjK-Ab...0/IMG_7074.JPG I know no technical terms so forgive me, but see the two pillars on either side of the cam stack? I noticed how the little finger is hooked into a slot that makes the pillar move up and down on the post? Both sides look the same on the above web image which was originally from: sewingmachinenut.blogspot.ca/2013/03/singer-401a-cam-stack.html
Ok, on mine, the pillar/post closest to the front, that finger has slipped out of the slot on the pillar and is beside the pillar. I expect this happened when I was trying to get it to move. I've tried a couple different angles to show it. On the other side, the finger is in the slot on the pillar.
I wonder if this is the missing puzzle piece as to why it suddenly stopped moving at all? Thanks in advance for any thoughts -