Imperf side varieties The stamps , and come imperf at top, or bottom. The right hand pair in the pane is imperf on the right side also, making 4 perforation varieties of this stamp in the pane. The left hand pair of the pane is perf on the left (the illustration is cropped at the left and appears imperf, but is not). Varieties like this are familar to collectors of Swedish stamps as they occur in many Swedish booklets.
Perforations The perforations are what they call "harrow" I think, that is the vertical perfs are done separately from the horizontal as many US stamps used to be. The result is that the center punching of a block of 4 will very likely have two holes punched that do not coincide. A well centered one hole punch will be scarcer and desirable. The closest here occurs between pairs 5 & 6. The other pattern which occurs that collectors like is a diamond of 4 holes. A nice one occurs between the 3rd and 4th pairs here. These also are scarce as the normal block just has some holes punched that don't match up well or make an attractive pattern.
Two Pass Perforating Finally the perforating device only perforates 5 stamps wide at a time, which means the perforations on the left side of the pane do not necessarily register with those on the right. In my pane the perfs on the left 5 pairs are very close to the design at the left, but on the right 5 pairs are right down the center between the designs. .
Stamp Size A second result of this method of perforating is that the stamps are not all the same size. Those stamps where the two perforating strikes meet will often be wider or narrower than the other stamps in the pane. This is how the famous "jumbo" examples of the US 19th century stamps are created. In the pane I have the 5th pair is narrower than the other pairs because the perforations are close on the left and well centered on the right.
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