I don't know if anyone still reads this blog. But if there is, for your information I can now tell you that I found the missing attendance lists from the first two weeks of the course. I had put them in a pile of paper that belonged to my other course, that I gave in parallel with this course.
It feels pretty futile to go through the lists retroactively to see if/who should have had to do an extra assignment. So, I have no use for them any longer and it wouldn't make a difference anyway as the final grades have been reported and can't be changed.
Still, a mystery that I was thinking about quite some "back then" has been solved.