I've noticed a problem with the file prefix "DUAL" on Dual ISO module.
On 5DIII with build 04 march, 22 feb and maybe before, when you set on the custom file prefix, it works only if your canon config for filename is set to "Preset code" and not if you set User setting1 or setting2.
So, my preset code is "AW9A" not very sexy !
Maybe there is a solution to force user setting with dual :)
Many thanks Tof