In all my reference material I have never heard of a T roof on a 240/260/280, but of course it's possible - I've learnt with American cars that plenty of exceptions to the generally published guidelines for models exist & are actually more numerous than one would ever think & not just prototypes - factories with car-construction resources are capable of creating all sorts of variations for senior staff or preferred customers or to showcase at special events. (There would have to be some careful modification to the roof structure in this instance). And there are, or perhaps were, also plenty of talented bodyshops around. Someone might have decided to emulate the 280ZX Targa that came out a couple of years later on their baby.
Ditto Rev, my very early 260 2-seat had flat-tops all as part of it's basic emissions controls. I have a book that states that the very last of the 240's in 1973 were similarly equipped which was one reason that the '71& '72's were preferred.
The 260 & 280z bonnet vents were a factory initiative in later years due to vapor lock problems reported in hot climates like Arizona etc, & elevated underhood temperatures thanks to more creature comforts like A/C being fitted. Lots of people have retro-fitted vents of various types for that reason.