All 7260s use the same drivers regardless of version (AC, N, dual band N). It's just that Dell decided that Intel 7260 AC is the device name for their drivers.
If device manager is saying you have dual band N, you definitely have dual band N even installing the AC drivers because within the 7260AC driver from Dell, all of the device descriptions that matches the hardware IDs is stored on a inf file. The point of inf file is to have 1 driver package for many devices out there. Think of NVIDIA where you can just download the GTX 780M drivers and it will install on your GT 750M graphics for example.