So here are some photos of the parts as they come apart from thumb nut to stud. I did not take out the stud or the check spring. I was not in the mood to take the whole thing apart. I should, though, and replace the check spring.
Push the numbered dial in and release the thumb nut. Screw the nut off completely and the numbered dial slips off.
to reveal the stop washer. Note the position of the stop washer.
I was surprised to see another stop washer.
To remove the whole assembly with the stud and the check spring attached, you loosen the set screw. The other larger screw is used to move the whole assembly within its housing, thus adjusting the check spring.
I believe that second stop washer is the Tension Release Pin cross piece that releases the discs when the presser foot is lifted.
That's it. Not all HA-1s are alike . I have a Brother that is different from this one. I believe the Genuine Singer is different as well.
The problem with this little project is that now I remember how much I like this cute little green machine. I may have to put it in the treadle stand. Sigh.