Way back in May (which seems ages ago) I mentioned a printing press called The Chambon. Basicly the main difference to the format of the sheets was these had a different sheet layout to the norm and were printed in double sheets of 100, the sheets were located one above the other, not side by side.
While I was going through some stamps yesterday I came across this Chambon Strip ( or block of 20 + labels as some may describe it)
You will notice the gutter separating the two stamps. To the left this has a warrant number and in the centre it shows the wording TOTAL SHEET VALUE and two arrows.
The arrows are designed to aid Post Office staff to separate the sheets and total up the remainder of stamps unsold during the days trading.
If you collect mint singles it is advisable to collect this issue as a gutter pair.
It should be noted that Chambon printings affects 2 stamp issues printed in sheet form, these are both 10p values. The first is a two band FCP/DEX with 10 mm phosphor, the other is printed on PCP1/DEX with additional two 10 mm phosphor bands.