Is there a mark on the back of part #4 - assuming that the spring is #3 - that looks like something other than a tension disk has been rubbing? Something smaller? Other than the release pin not having something to stop against - it looks almost right...
In the Riccar photo - assuming that the spring is 3rd from the left for reference purposes and the tension disks are 5 and 6 - I think the order goes:
1 inside 2, 3, 5,6, 4 (and that finger will nestle into the next part - preventing movement of the +|- dial), 7, 8, 9, 10, 11.
Does the new tensioner have that cross piece inside the +|- piece, like a Singer tensioner?
I'm trying to think of something that could be substituted for that release pin stop but I'm coming up blank. I had a Singer tensioner that was broken there and tried lots of things but in the end, had to borrow that piece from another tensioner. It might be worth asking the lady if she has a drawer of "spares" where she puts things she finds... and check it.