bounding box docs and semantics are confusing about inner margin
With the bounding box option for Avery 5260, I saw vertical space between the boxes, but the labels do not have any vertical interlabel space, either physically or in the label definition.
Investigating, I see that there is an inner_margin
parameter, set to 1 point, that results in the bounding box being 1 pt inside the defined label size. This explains the space which I think is 2 points.
Probably, the concept of margin should be applied to where text is printed, rather than to the clip region, sort of like how in TeX one can print on the whole page, but using the other point is a bad idea because of registration accuracy and because some printers have unprintable edges.