This is very ingenious– finally someone figured out a way to add a pen loop to a notebook without actually adding anything!
Front:

Back:

Perhaps having to un-do 2 elastics would get tiresome after a while, but I still think it’s clever!

For sale at Westerlind: Makr – Sketchbook.

UPDATE: Looks like it’s not available at Westerlind anymore but you can get one here.