Specialised stamp catalogues list three sizes for this issue but refer to slight variations for different usages i.e. sheets, coils, booklets. Some writers have listed a multitude of sizes, which has always puzzled me, and I decided to try to resolve this problem. Unfortunately information in Post Office files is difficult to find and it has been necessary to trawl through a number of files to get to the facts.
It must first be made clear that apart from one minor change affecting sideways delivery coils (which I will come to later) the area allocated to each definite stamp had remained constant for many years at .8 x .95in with a standard gutter perforation margin (the distance between each stamp) of .0625in (1/16th). The stamp image size is the area size less the perforation margin. These measurements were those used for the first (Large Format) photogravure stamps. Because of the well known perforation problems the gutter margin as quickly changed to .075in (Intermediate Format) and this measurement applied to both sheet and booklet stamps. When this still proved insufficient the final reduction was made with the gutter margin being increased to .095in. Again this size applied to both sheet and booklet stamps.
Coil stamps proved more of a problem. The machinery was new and numerous modifications were necessary. As an aside in one report, it was mentioned that the Stationery Office were experiencing difficulties with a similar machine for the production of surface printed Insurance Stamp rolls. They too were forced to go back to Waterlow for supplies in the interim period.
The diameter of a standard photogravure cylinder was 19.95in. For end delivery coils there were twenty-one stamps (21 x .95 = 19.95) round the cylinder and twenty-four across.
After initial trials, authority was given on 19 October 1934 for an increase in the width of the gutters from .0625in to .07in (I have not found any reason why gutters were not changed to .075 as for the sheet and booklet stamps), and by the beginning of January 1935 this had been changed to the now standard Small Format gutter of .095in. It has long been known that the first printings of Rolls E and L were of an unusual size and with a gutter margin of .07 this would give a stamp image size of 18.542 x 22.352mm. Records however show that up to the end of 1934 Harrison had delivered the following rolls:
It is therefore possible, although not proved, that these could all have been in the 'odd' size and that therefore rolls K may also exist.
Sideways rolls had a different problem. To fit on the cylinder twenty-five stamps were placed sideways round the cylinder with twenty, in two blocks of ten separated by a margin, across (twenty-five x .8 = 20in). Easton (Postage stamps in the Making page 231) suggests that a 20in cylinder was used but this is not borne out by the facts. The printers overcame this by reducing the total stamp area by .002in to .798 x .948 (25 x .798 = 19.95) and the gutter margin by the same figure to .093 leaving the stamp with the same image size as the Small Format stamps.
Stamp sizes can therefore be summarised as follows, stamp area .8 x .95 in all cases:
Sideways delivery coils, stamp area .798 x .948in
Intermediate Format includes both sheet and booklet stamps. Small format includes sheet, booklet and end delivery coils.
And those are all the official sizes found in Post Office records.
Why then have so many different writers given different figures for these stamps. Some can simply be explained by difference in rounding up or down. There is also the problem of accurate measurement as photogravure printing by its very nature tends to have irregular edges. There is also the possibility of expansion/contraction of the paper. Although this normally occurred with printing on dampened paper it could occur during dry printing and gumming as is evidenced by a report on some paper from Portals (John Allen) sent to the Government Laboratory in 1934 for testing which includes the following, which would be more than sufficient for some of these differences:
Ref PO 33/4972 PO 52/527 PO 52/988