Yep, you're right Hibi. 'The Unlocked Window' episode was indeed first broadcast about half-way through the hour-long series' final season of 1965.
The very final series episode being one that has John Gavin cast against type as a homicidal cop, titled "Off Season".
And, according to Cotten's IMDb filmography, he starred in 3 of the half-hour episodes, but not in any of the hour-long ones.
(...his most memorable one for me being the one in which he's paralyzed in a car crash and everyone thinks he's dead, titled 'Breakdown'...first telecast in 1955)