I bought a MK25, which came with 3 mags, and bought another 2 regular 226 9mm mags NIB from a guy on here.
I cannot see any difference whatsoever, they look exactly the same inside and out, they don't have that rainbow-colouring that some of the gun parts have from the phosphate, and they all have the same minor scratches after a while of use.
I contacted SIG about this, they said that the gun was phosphate-coated, but the magazines were not.
There's 3 different answers floating around out there...just my 2 cents.