It looks like it may be complete and even has the tension release spring. I looked at a couple of videos
https://www.youtube.com/watch?v=xK86anBrBsc and
https://www.youtube.com/watch?v=muCDrG9Ii3c but can't quite tell for sure whether it is complete or not. According to the manual there is supposed to have a "check spring arm notch (C)" but I can't quite tell from the scanned model and I don't see it in the videos. I just a little closer at the first video and he describes threading it and it looks like there isn't anything that the thread needs to catch on on other than the actual "U" section of the check spring.
It looks like the set screw is there as indicated by the red arrow. I would try to unscrew that. You will need to put the check spring on the back so that the "U" section is open on top before you put the tension assembly unit in the machine. place the unit in the machine and then tighten the screw. Gently tug on it and see if it comes out again. If so then there may be a missing piece.
As a side note about posting pictures I took a screenshot of the
enlarged window for attaching images. Sometimes it takes awhile for them to load but you want to have it show the "Current Attachments" as indicated by the red arrow, before you close that window.
Remember to click on thumbnails to see full size images. I usually right click and open in new tab, as I have a tendency to close that window and then I have to go back and find the post that the picture was from LOL.