OK, I feel the need to jump in, as I've never liked the term "slabside".
To me, it's not something that sits comfortably with an early (260/289) Cobra. I have always used the term "roadster".
So....I grabbed the easiest-to-hand relevant publication on my desk, the Brooklands "Shelby Cobra Gold Portfolio, 1962-1969" and I turn to the first road test there,
which is John Christy's test of the 260 Cobra. Christy describes the car as "the pearlescent yellow roadster". No mention anywhere of "slabside".
So, "roadster" from day #1
Cheers!
Glen