Woohoo!!
#5020698 rides again!
I'm positively giddy.
The lens is back together and it works! It's not good as new, but it's a lot better than it was a few hours ago.
Talking it out here helped me to realize I was making the job harder than it needed to be. When I finally got the mount parts reoriented properly, the process went smoothly. And you guys were right
: When I did get the spring attached, the tension was enough to hold it in place.
I used a dental pick to hold the spring while I fitted the ends over the posts / studs. Here's an example of what it looked like (this is a different spare spring):
And here's the actual spring in its new home:
The replacement spring probably isn't as strong as the original because the iris is a bit slow to close down. And it won't stop down beyond f/13. But that's still a lot better than being permanently wide open!
Actually, the lazy iris doesn't matter because this is no longer an auto stop down lens anyway. A former owner cut off the rear aperture lever - likely to avoid it hitting the mirror on a Canon DSLR. No doubt the mount was disassembled in order to try to remove the lever. And that's probably when the original stop down spring went missing.
Unfortunately, there's no (simple) way to non-destructively remove the aperture lever.
So it's a hobbled lens. But I'm sure it'll still be capable of capturing beautiful images.
Many thanks to Adam, Ole and the members here! Without Pentax Forums, I doubt I would've succeeded.
This was a lot more fun than getting socks for Christmas!