I have written about Blumenberg and his critiques of Luhmann before. Here is a picture of what his Zettelkästen--yes, he was German, even though he had every reason not to be--looked (look) like.
Click on picture to enlarge!
My Zettelkasten for the dissertation that I wrote between 1996 and 1999 looks somewhat similar, though I used a smaller format (DIN-A 7: 74mm x 105mm). I promise I will post a picture after the holidays. It's archived in the office.