We Three Kings

 Karl II, Ferdinand I and Karl I appeared on this stamp issued Nov. 1, 1931 commemorating the 50th anniversary of it's elevation to Kingdom from Principality. Romania's independence from the Ottoman Empire was recognized in 1878 by the Treaty of Berlin.
 As to why and how the two Karls got on this stamp I have no clue. If your interests include Coat of Arms, they appear to border the vertical edges. This stamp was also printed in a 10 Lei value of a blue color vs this blue green hue. Also the 10 Lei stamp had the years 1881 and 1931 appearing in the dark rectangles on the bottom left and right of the portrait. Someone must have forgotten to put the dates on the 16 Lei issue.  

JimmyB