There is a black (or now gray on newer boxes) rectangle on bottom of 240XL and 241XL ink cartridge boxes (e.g., 4914D16, 3913G16, 2A14E25, 2A14C24). The solid rectangle is to the right of the bar code. No one seems to know what the number means, including Canon (said it was inventory control, but I believe that's what the bar code is; maybe not .....)
Thanks!
