Designed for Coptic stitch bookbinding technique
Workable area is 8.5", perfect for Letter (8.5"x11") sheets turned sideways, markings are imperial with 0.25" (6.35mm) spacing on center of each 0.13" (3.3mm) hole.
Markings are recessed enough to be distinctive but can always dab some paint into the markings and wipe across the top with a credit card for a professional, contrasting effect.
2023-01-27 EDIT: I've upgraded the guide block for better tolerances
The author marked this model as their own original creation.