Crystal Knight Project
After a survivor was found at the site of a crashed UFO in New Mexico (see 'EBE'), he was allowed to contact his species, and a programme was launched for the exchange of ambassadors: eventually twelve people were sent for 13 years to Serpo, while three "Ebens" (some sources only mention one) stayed on Earth. The name of this exchange programme allegedly was Project Crystal Knight. It ran from 1954 until 1978. A first attempt to exchange ambassadors failed in 1964. A second attempt in 1965 was successful and lasted until 1978. See 'Serpo' for more information.
Note that while most sources agree that such a programme has indeed existed, it is only in the Serpo material that the name Crystal Knight has been mentioned for this programme. As much of the Serpo material turned out to be unreliable, the reliability of this information, too, is doubtful.