I have never heard of anything called the Serenissima 308 Jet Coupe.
According to the UCP blurb,

"Under a clear blue sky, the Grand Hotel Villa d'Este hosted the annual
Concorso d'Eleganza this past Saturday. First organized in 1929, it is
one of the oldest concours d'elegance in the world and as such has a
number of unique characteristics. One of these is the restricted access
to 'friends of the concours', the entrants and the media. Public were
welcome at the Sunday Villa d'Erba show. The number of entered cars was
limited to just 49, ranging from 1914 to 1974 in age, all of which were
of particular interest. Many were freshly restored or brought out of
hiding for the first time in many years. One of the bigger surprises was
the Serenissima 308 Jet that had not been seen for forty years and had
many of the visitors grabbing their catalogs to figure out what it was.
In addition to the historic vehicles there was a class dedicated to
modern concept and prototype vehicles. The Ferrari 575 GTZ and Russo
Baltique Impression Concept made for two impressive world premiers."

http://www.ultimatecarpage.com/gallery.php?num=2441

Btw imho that 575GTZ concept car is hideous, as is the one based on the
612 Scag.  I hope Mr. Glickenhaus' idea was conceived with a bit more
beauty in mind.