The "591" is the same as the "590" apart from the tiniest difference in staple size (not visible to the eye). Why make the "591"? Because this slight difference makes it compatible with standard #10 staples in Asia.. so these are "staplers for the Asian market". Why do we have them? Because the colors are so cool and they're not available in the standard "590" (which, again, is basically identical..)

Light, efficient and easy to use. ABS 'anti-shock' plastic top. 200 staple loading capacity. Painted with lead-free enamels with high resistance to wear and corrosion.

staples 15 sheets with included staples
holds 200 staples through top-loading system (not 'plunger' style)
1.81" depth (46mm)
boxed; includes one box of 1000 130/E staples
patented anti-jamming device

staples: order refill Z-110