Marked so as to be recognised. The allusion is to marking cattle and sheep on the ear, by which they may be readily recognised.
“The increase [of these wild cattle] were duly branded and ear-marked each year.” —Nineteenth Century (May, 1893), p. 789.
“The late president [Balmaceda] took on board a large quantity of silver, which had been ear- marked for a particular purpose.” —Newspaper paragraph, Sept. 4, 1891.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894