I read from this article that
The discovery of the Fourteen Canons (BWV 1087) on the inner side of the back cover of his own copy (hereafter referred to as the Handexemplar ) of the Goldberg Variations (BWV 988) made Bach's fascination with these aspects even more apparent.
I don't really understand that. Was the back cover of Bach's copy thick enough to contain all 14 canons? Or were they written inside the copy that's near the back cover?