Not your imagination, I've noticed the same thing about the clips. Comapare the Arrow to the Canon and what you'll find is that the beveled edge on the bottom of reservoir end of the tank is a bit wider on the Arrow than the Canon. This makes it easier for the Arrow tank to snap into the acrylic...
The BCI-3ebk is used when in plain paper mode (often referred to as text printing). On my iP4000 I've also tried switching tanks to come up with different shadings of black. As I remember, the black still looked the same and appeared to have been solely from the BCI-6bk tank. I on the four tank...
Rob, I finally registered for this site. It's been a long time since I sent you the quick and dirty pdf file on refilling BCI-6's.
Are you really sure about the above cartridges. I compared my Arrow tanks to my Canon OEM tanks and they are exactly the same size.