I picked this machine up last weekend. When I got it, the spring (below) was sitting in the bottom of the arm. It didn't do a couple of the stitches correctly, because it was missing some tension on the bar by the cam stack.
I tried the spring in various orientations, but most of them just resulted in me getting hit with a spring.
In this position it stays, but I'm hoping someone here might have a hint as to where it actually goes.
I've since slid it to the back of that bar, which lets it sit a little flatter on the left side.
With the spring in place, it does stitch just fine in all of the stitch settings.
One side of the spring has a rounded hook, the other side is squared. I couldn't find any holes to put the ends of the spring through, which I thought was weird. Sitting it where it seems like it can get bumped seems like a recipe for,.. well a lot of service calls, which is why I'm not sure it's where Elna would have put it originally.