Sergi,
I received a card from 4U1UN for a 10m contact on Oct. 24, 1998.
The card was stamped: 4U1UN
PO BOX 3873
NEW YORK, NY. 10163
The card itself had printed on it: UNITED NATIONS AMATEUR RADIO STATION
UNITED NATIONS
HEADQUARTERS
BOX 20
NEW YORK, NY. 10017
I sent my card to the Box 20 address.
Hope this helps.
de Bob k5sit 73