Although I can't be sure (a closer scan would be helpful) the cancel date appears to be April of 1918. Even if it is 1919, the EDU for #539 is 6/30/19, and Max Johl give an issue date of 6/14/19. Is there any censor marking on the back? Since WW-I was still in progress, shouldn't the cover be censor examined? What is the measurement of the design? It doesn't look "square" enough to me. Also, a scan of just one of the two-cent stamps might help in identification.