The following is quoted from Stanley Gibbons Great Britain Volume 4 Queen Elizabeth II Decimal Definitive Issues
The Machin sheet stamps are printed from double-pane cylinders of 20 rows, each sheet being separated in the reel by a stamp-size gutter.The sheets were divided by a rotary knife. Occasionally a sheet missed the rotary knife and had to be guillotined along the perforations, giving rise to stamps se-tenant vertically with blank labels but containing the marginal bars.
For sheet stamps the perforating cylinder, or drum, has 48 rows of pins. On its introduction in 1973 one of the rows of pins had an extra pin on each side giving an extra extension hole in the left margin of the no dot sheets and in the right margin of the dot sheets.
The purpose is to make it easier to locate faulty pins, hence it is called a marker pin.
The pin on the left side disappeared late in 1975, reappeared early 1977 and disappeared again in the autumn of 1978. It is assumed there are two drums, one without the left marker pin, and that each is periodically taken out of service for repair.
Early in 1980 two further pins were added to a drum to give a single extension hole in both sides of the no dot and dot panes. With each revolution of the perforating drum the extra extension holes appear in a different position in the second or third sheet in the sequence and as there can be either two or three stamp-sized gutters within a single revolution the extension holes occur under both even and odd numbered rows.
There are three possible sequences (or Setting) depending upon the starting position of the drum as follows….
Thus if all the settings are used the extra extension holes can be found in every row of the sheet, but the three settings are not always used for every listed stamp.
So far as cylinder blocks are concerned, Setting 1 gives the extra hole on no dot blocks in row 19, Setting 2 in row 20 and Setting 3 in rows 18 or 21. These are, of course, collectable items and worth a premium but since they are not representative of a different perforator we do not list them.