After a survivor was found at the site of a crashed UFO in New Mexico, 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 “Ebans (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.
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.