Triangular Cancels. 
In England and Ireland before 1855, newspapers and similar printed materials passed through the postal system free. The cost of their transportation was indirectly covered in a tax, which had been levied since 1712, and was denoted by a red stamp impressed on the front page of the paper. By 1855, this tax came to be viewed as a tax on knowledge. The tax was made optional, and newspapers not showing the tax stamp were transported at Book Post rates, which was inexpensive but offered slow delivery times.
In 1870, the Post Office adopted a second class, halfpenny rate for newspapers, circulars and other printed matter. This change spurred massive growth in printing and circulation of newspapers. While service still was not time-sensitive, it was so cheap that it permitted publishers of even small-run papers to thrive. To qualify for the printed matter rate, items had to be printed.. By 1895, typewriters and duplicating machines were coming into wide use, so the Post Office enlarged the criteria to include items reproduced from typewriting. All printed matter had to be sent in unsealed envelopes in a minimum bulk number, and had to be deposited at the head office or one of the larger branch or sub-branch offices for cancellation in a distinctive matter.
Triangular handstamps for canceling newspapers were provided to major city and branch post offices and their marks appear on wrappers starting in about 1895. The first triangle cancellation devices bore the telegraph code of the processing office, using a two-letter or three-letter abbreviation. Two sizes of triangle handstamps are known; the smaller size sometimes has punctuation.
Often, the telegraph code provides no hint about the name of the office with which it is associated; Location codes sometimes changed over time. When Southern Ireland was a British possession, Dublin used the two-letter code DN until 1922; after adoption of the Irish name Baile Atha Cliath, the code was altered to BAC (Figure 1). . A table of Code letters is shown below.
Triangles may be found from a few offices which show their three digit type office number (obsolete after 1906) in combination with their telegraph code letters. An example is CTV / 104 (Carrick-on-Shannon). (Figure 2). After 1922, separate codes were used to distinguish Northern Ireland from Scotland and the Free State. Only major offices received a number. For example, Belfast was allotted the code I.5; formerly it was number 62.
Triangle devices also were used in machine cancellations. Dublin and Cork are known to have machines. Belfast had two different machines and may have been unique in Northern Ireland in having them. Marks with wavy line and slogan cancels are known. Cork used a CK die, which may be seen with wavy lines and even slogans. Dublin had three machines, “Columbia” (used 1906-1914 and 1915-1919). The second machine was the Hey-Dolphin and examples of B.A.C. in triangle with a wavy line obliteration are known. The third was a Universal which shows B.A.C. in triangle with a broken 3-part wavy line obliteration. (Figure 3).
Use of the triangle cancels continued for a remarkably long time; examples from the 1960’s are known. In 1968, Northern Ireland introduced a two-tier rate system that made the triangle stamps theoretically obsolete. However, triangle stamps could still be used to cancel stamps that had missed normal machine obliteration or for use when unsealed printed matter envelopes were examined to see that they conformed with the regulations. Cancel examples have not been found used after 2003.
The total number of triangle cancels used in what is today’s Republic of Ireland is not large., as listed below. Use of the triangle cancel continued into the early 1980s, so these markings can be found on virtually all definitive series of Irish stamps through that period, and less commonly on commemoratives. This is a challenging collecting area since most was used on mail that is not often found in dealers’ boxes. (Figures 4-9)
Irish Postmarks Since 1840 , Mackay, James A., pub. by author, Scotland, 1982, pp. 112-114.
The Triangular Marks of Great Britain, Layne,Harry, pub. by the author. 2003, pp. 130-137.
C. &, The Postmarks of Great Britain and Ireland, Being a Survey of British Postmarks from 1660 to 1940, Alcock, C. & Holland, F.C., pub. by R.C. Alcock Ltd, England, 1940, pp. 385-399.
London Postmarks Used on Printed Matter and Parcels From 1860: A Guide for Collectors, Fernau, C., GB & Commonwealth Philatelic Society (Switzerland), 1998, pp. 21-22
“Irish Triangular Cancellations”, Leonard, M., The Revealer, Jan-Feb 1954, p. 152, 181
N. Stack, “Triangular Cancellations,” The Revealer, Mar-Apr 1954, p. 158
“Triangular Cancellations of Great Britain”, Peach, M., The Canadian Philatelist No. 6 (Nov/Dec 2002), pp. 34-35.
Collect British Postmarks, Stanley Gibbons, Ltd, Ed. Pipe, Bill, 2013.
Compliments of Chris Shapiro. 12/23/2018