As there are some dead links and missing illustrations from the earlier threads, this information may be of interest:
"Henry William Cave came to Ceylon in 1872, at the age of eighteen, to work as the Secretary to the Archbishop of Colombo. Four years later he established a bookstore. Soon the business expanded its merchandise to include musical instruments and eventually a wide variety of other consumer goods which found a ready market amongst entertainment hungry planters. Cave returned to England in 1886, the year in which this stamp, bearing the imprint of his company was issued. However, he continued to be interested in H.W. Cave and Company until his death in 1913.
These private overprints, similar to private "perfins," were intended to prevent theft of stamps for non-business use by employees. They are the private equivalents of various official overprints such as "On Service". "
Also: The overprints were applied between the years 1884 and 1908. Overprinting ceased in 1908 when the company instead began to use the 'CAVE' perfin.