One of my favorite songs in '66--at age 7--was WINCHESTER CATHEDRAL, a massive radio hit that played into the earliest part of the big nostalgia boom of the era with megaphone vocals and old fashioned...well, everything.
It would be many, many years later before I found out that the group that had this massive favorite hit...didn't actually exist.
Although credited to The New Vaudeville band, it was expected to be just a novelty release and so the Producer just went with the demo vocal, a practice that happened fairly often in those days. The record was made by studio musicians built around that demo vocal.
But then when it became a hit, some musicians were hired to form a real band and a singer was hired who could sing like the record.