Surcharges imposed by the Irish post office on mail bearing insufficient pre-paid postage had the postage due collected by the use of these labels. Since 1925 there have been six series printed, with the design remaining the same until 1980, though the colour and watermarks have changed. The Irish word "pingin", for penny, is used in both £.s.d and decimal currency, but because it has the same meaning in each, the value on the label does not indicate whether the label was issued before or after decimalisation. Hence, identifying the issue of a label requires further information: if collectors knows the date of use, the existence of a watermark and if so which type, and the specific colour, identification will be easier. For example, the 3d value was blue between 1940 and 1969, and stone colour from 1971 until 1980; it changed from a watermarked to a non-watermark paper in 1978. Additionally, the 1, 5 and 8 pence values are seen in two different colours depending on the issue, while the d, 1d, 2d and 6d are seen with both watermark varieties. Except for the sixth issue, which has the word Éire in the design, there is no explicit country identity on any of the others. The first four series use only Irish words.