The 1st July 1875 saw the most radical postal reform for Universal Postal Rates going overseas and countries throughout the world gradually joined this reform, with Great Britain being among the first.
On the 1st July 1875, the General Postal Union fixed the cost of sending, for all member nations, at the deficiency of postage plus twopence and a halfpenny per half ounce. This regulation proceeded into the Uniform Postal Union, effective 1st April 1879, and changed to be twice the deficiency.
The illustrated covers above are two lovely examples of how the twopence was paid (by stamp and stationery cover) and the ½d. rate additionally applied. Details of these covers can be found here.
A special 2½d. Rosy Mauve adhesive was printed to service this new rate, on 1st July 1875, as plate 1 (watermark anchor) with successive plates to Plate 17 in December 1879 (watermark orb).
Below is another example of this new rate, with printed, green halfpenny wrapper, together with the twopence halfpenny stamp, making a 3d. rate, which serviced 6 ounces at the printer paper rate. Details of this item can be found here.