Service on alien property custodian.

In every action or proceeding in any court or before any administrative board involving real, personal, or mixed property, or any interest therein, when service of process or notice is required or directed to be made upon any person, firm or corporation located, or believed to be located, within any country or territory in the possession of or under the control of any country between which and the United States a state of war exists, in addition to the giving of the notice or service of process, a copy of the notice or process shall be sent by registered or certified mail to the alien property custodian, addressed to him or her at Washington, District of Columbia; but failure to mail a copy of the notice or process to the alien property custodian does not invalidate the action or proceeding.