Technically speaking I think it started with the 1750 GTV in 1969 (?). This is because the 1750 shell has modified rear seat and roof shape to increase the passenger space.
Shells before this are different, but so similar I cannot tell the difference
Otherwise you would have to go right back to the first model with the smooth (instead of stepped) front ... not even sure what model that was.
ps: Remember it's all just opinions
'71 1750 Series 2 GTV: http://www.alfabb.com/bb/forums/showthread.php?p=208078