  In the winter of 1967 Cambridge radio astronomers discovered a new type of
radio source of such an artificial seeming nature that for a few weeks some
members of the group had to seriously consider whether they had discovered an
extraterrestrial intelligence. Although their investigations lead them to a
natural explanation (they had discovered pulsars), they had discussed the
implications if it was indeed an artificial source: how to verify such a
conclusion and how to announce it, and whether such a discovery might be
dangerous. In this they presaged many of the components of the SETI Detection
Protocols and the proposed Reply Protocols which have been used to guide the
responses of groups dealing with the detection of an extraterrestrial
intelligence. These Protocols were only established some twenty five years
later in the 1990s and 2000s. Using contemporary and near-contemporary
documentation and later recollections, this paper discusses in detail what
happened that winter.
