T IS FOR TAXE. 
Postage due stamps are an indicator to a postal delivery person that there is inadequate postage on an item, and that he or she must collect the difference.. On international mail, a rubber stamp with a “T” in black ink indicates postage is deficient. This marking, or a similar one, has been used in Ireland since 1875. Mackay shows many types of stamps used indicating postage due, but the “T” is limited to international mail. (Figure 1). Some are simply a “T”, but others contain the amount deficient and in some cases, the telegraph code (DN = Dublin, BE = Belfast).
Any shortfall is calculated under U.P.U. Article III. “The charge on unpaid letters shall be double the rate levied in the country of destination on prepaid letters.” In short, the sending country must determine the shortfall, and double it. Another regulation notes “… When an article shall be insufficiently prepaid by means of postage stamps, the despatching office shall indicate the value of the postage stamps [on the cover] in gold francs and gold centimes.” In operation, this meant the sending country express the total shortfall in centimes. It is the responsibility of the receiving country to convert the deficiency (in gold centimes) to local currency, apply postage-due stamps and collect the difference. (Figure 2). In this case, the US Post Office has converted the 10 centimes to 2 cents, and applied postage due stamps. (Figure 3).
The gold franc or centime was a rate established for international transactions which converts one currency in terms another. This was not an actual currency as such; rather it was a conversion factor. The conversion factors changed as the value of currency. In the period of hyperinflation in Germany, the conversion changed significantly daily. In 2003, the gold centime was replaced by the International Drawing Right, since most countries were no longer on the gold standard, many since WWI.
The cover shown in Figure 4 was correctly marked as deficient, but the deficiency was not collected by the U.S. Who pays? The U.S.would sine Ireland would have included this as a credit when reconciled.
The rubber stamps used were normally limited to exchange post offices; that is, those dispatching mail directly to foreign countries. In Ireland, initially Dublin, Cork, Londonderry and the Dublin-Cork TPO were the only offices that sent mail to ships. Belfast was later added. Of course, this changed greatly with the advent of airmail. Computerization and changed rates have basically rendered this obsolete.