According to Roy Schneider's book, Cadillacs of the Sixties, 1964 saw the debut of the new name, 'Fleetwood Eldorado'.
1963 was the last time 'Eldorado Biarritz' would be used until 1977 when it was used once again but only to designate a trim package. From '64 thru '85, Eldorados were known as Fleetwood Eldorados. There were Biarritz models but as in '77, the word 'Biarritz' referred to a posh trim level. The cars themselves were part of the Fleetwood designation.
HTH...
Greg McDonnell
CLC#20841