The article on the Admiral issue by Randall W. Van Someren on the BNAPS web site includes a chart that summarizes the postal rates during the Admiral period. See:
http://www.bnaps.org/ore/VanSomeren...ps-Rates.htmThe 4c stamp would be a natural for at least two rates:
1) The UPU postcard rate from 1 Oct. 1925 to the end of the Admiral era. An example of this rate is shown at the bottom of the chart. It is a postcard mailed from Quebec to Russia on 21 July 1927.
2) The British Empire first class rate from 1 Oct. 1921 to 30 June 1926. The most common destination is Great Britain, but shown below is a cover to Malta postmarked 26 Jan. 1926.
Bujutsu, the cover you show corresponds to neither of the above rates. It was mailed to the U.S. on 4 Jan. 1928. From the rates chart, this would correspond to the U.S. and Mexico first class rate for a letter weighing from 1 to 2 oz. The rate came into effect on 1 July 1926. I suspect that examples of this rate are scarcer than the two rates mentioned above.
