Coded postal obliterators are a type of postmarks that had an obliterator encoded with a number, letter or letters, or a combination of these, to identify the post office of origin. They were introduced in the United Kingdom in 1843, three years after the first stamp was issued. They became common throughout the nineteenth century but very few remained in use until the twentieth century.
The practice of cancelling a stamp on a letter with a device to prevent reuse and applying alongside, or on the reverse of the article, a dated stamp including the post office name, began in 1840 when postage stamps were introduced in the United Kingdom.
Soon, in many countries, various systems developed where the obliterator had a code identifying the post office of origin. Most such codes were enclosed within a design of dots, rays, bars or concentric circles or ovals to ensure the effective cancellation of the stamp. Similar designs without an enclosed code are known as dumb or mute obliterators. Later the duplex canceller with the datestamp on the left and the obliterator on the right came into common use. Coded obliterators were used throughout the nineteenth century but few persisted to the twentieth century.
Coded obliterations are collected by philatelists and rare examples can command high prices.
Postal administrations which used coded obliterators include:
Penny Red with a numeral "70" postmark of Boyle within the diamond shaped cancel as used in Ireland
The first coded obliterators were numerals within a Maltese cross design used at the London mail centre from 1843. From 1844 distinctive and different barred designs were introduced for England, Scotland and Ireland. Codes corresponding to London postal districts e.g. W21 were also used.
Jamaica used the barred obliterator "A01" for the capital Kingston, originally on the stamps of Great Britain. The obliterators "A27" to "A78" were used by various district post offices, once again also originally on British stamps.
Malta used the barred obliterators "M" and "A25" (both Valletta), initially on British and later on Maltese stamps:
The "M" postmark was introduced in 1857 and remained in use until 1860. This cancel is only found on British stamps and the initial printing of Malta's first stamp, the Halfpenny Yellow.
The "A25" cancel was introduced in 1859 and it remained in use until 1904. There are several distinct types of this cancel since it was used for a long period of time. "A25" cancels are mainly found on British and Maltese stamps, and also on some foreign stamps due to maritime mail.
The obliterator was as a diamond or trapezoidal shaped grid of dots, measuring about 20 millimeters (0.75 inches) on each side, with a set of numbers at its center. The number consisted of anywhere between one and four digits, and was unique to the post office that used it. For this reason, such numbers are sometimes referred to by philatelists as "town numbers."
Two main types of the losange were used, the "losange à petits chiffres" (small numbers) from 1852 to 1862, and the "losange à gros chiffres" (large numbers) from 1862 to 1876. Distinguishing the two is based largely on the size of the numbers and the size of the dots in the cancel. The dots are noticeably smaller in the "losange à petits chiffres" and its numbers measured approximately 4 millimeters high. Town numbers between 1 and 4494 are documented. Town numbers from 3704 to 4018 and 4222 indicate use at a French Post office abroad.
The "losange à gros chiffres" obliterator had numbers twice as big being nearly 8 millimeters tall. Since the overall size of the cancel remained the same, a larger portion of a large number obliterator consisted of the town number. The list of numbers used increased as the number of new post offices opened, with numbers known up to 6449. Town numbers 2387 and from 5079 to 5156 were those assigned to French post offices outside continental France.