In the early 1900s, the Swiss started to print special sheets for booklet panes. These sheets consisted of rows of 10, and the layout was as follows: three stamps upright, an empty stamp-size field, three stamps upright and finally three stamps inverted. Now, if you picture this in your mind, it's clear to see that these sheets yield both tête-bêche pairs and gutter pairs. And it's the gutter pairs which I'm showing you today.
The empty stamp-size field was prone to be used for the forgery of stamps, so the Swiss PTT had to do something with those field. That's why they decided to punch a hole through them. Now, the thing with these holes is, that they exist in circular form, and in not really circular form. This one here, like the first image, is of the not circular type.
And here is a more carefully produced hole, lovely and circular.
I wonder whether more than one perforator was used, with either a circular or not circular pin, or whether the not circular hole was just a matter of deteriorating material. The latter could well be the case for later, a smaller hole was punched through the paper.