This is a cover I have had for some time.
Apparently Box #252, New York City is significant. From what I have read in the past, this was a box that was allocated by the G.P.O. in NYC for belligerent countries during WW 2 for mail to go to the United States. At the time this cover was posted, the U.S. was not yet in the war. Both the Allies and Axis powers sent mail to this box. I believe it was used as a 'liaison' connection (?)
From my understanding, most of the covers required 90 Pfennigs to one Mark postage. Few are known with less as far as I know. I heard that fewer are known with more postage. This is unknown at present.
Anyway, this is a scan of a cover that I have addressed to box #252.
Chimo
Bujutsu
