Diamond Cancels, Numeric Diamonds. 
In 1844, diamond shaped cancels with numbers in the center were assigned to important post offices. (Figure 1). Ireland originally had 450 numbers assigned in alphabetical sequence. The list was revised in 1856,1874, 1887 and 1905. The final total was 564 which was only partially alphabetical. Some cancels remained in use until the mid-1920’s. When new numbers were added to the alphabetical list, it was sometimes necessary to reassign numbers, thus some locations had different numbers at certain times. For example, 279 was assigned to Killeshandra and later Killyles. The alternative of assigning newer locations open numbers gave Adara 561 instead of being a low number with other “A” locations.
These cancels can be viewed in a type sequence. The design was symmetrical, both horizontally and vertically. That is, the number of bars used to make the diamond shape above the number is the same as the number below, and similarly to the left and right. Interestingly, 18 of the 25 possible combinations were used. (Figure 2). Since these cancels were handmade individually from brass, errors occurred. Figure 3 is a cancel from Limerick with eight parallel lines in the triangle above the number but only seven parallel lines below. Figure 3A is the cancel with the stamp image removed.
Initially, the diamonds were used by themselves called numeric cancels. Later, they were combined with a circle with the clear-text town name and the date to the left of the diamond, and are known as spoon or duplex cancels. (Figure 4). Some cancels combine the diamond and the circle by replacing the horizontal lines on each side of the number with vertical arcs that compliment the circle. (Figure 5). Other varieties exist with the town information in a skeleton form without an outer circle or in a diamond-shaped box. A modern version of this was used in 1963 to commemorate a meeting of the Postal History Society.(Figure 6).
Collect British Postmarks, Edited by Colin Peachy, Stanley Gibbons, Ltd, 2013.
“Postmarks”, Dixon, F.E., The Revealer, Supplement #26, January 1966, p.115.
“The First Numerical Cancellations of Ireland” The Revealer, Nos. 58-64. A series of letters between Dr. Brian deBurca and William Kane discussing these cancels. Also commentary by William Dixon.