Kind of very big label (or, sometimes, specially printed sheet of paper) for wrapping up (initially) the dozen of boxes.
Now the term is also used for a wrappers of ten, six or other reasonable number of matchboxes.
Often consist of three printed panels - 2 short (for each end) and 1 long for the top of the package.
"A printed sheet intended to wrap a dozen packet - a packet containing several matchboxes. The term is derived from a packet containing a dozen (12) matchboxes but is also used to describe packets containing ten (10); a half-dozen (6); or three (3) matchboxes." (definition from the BML&BS site)
Equvivalent Russian term Этикетка пачечная
From collection of Victor Raldugin