I don't know much about NCOWS but there is no white plastic spacer in the Stoeger Supreme... just black rubber against wood. Nor does it have a vented rib. That being said, it is not on the "Unapproved List".
The stainless receiver is polished (nickle-like), so it does not fall astray of the no matt or brushed finished rule, but again, I don't know much about NCOWS, so I defer to Major 2.
There is also a genuine nickle barrel / receiver combo version available, but to me it just looks like too much poser shiney-ness to nickle plate the barrel. If all that fails, there is a Supreme version with blued steel everything, a thick rubber butt pad, and once again no white plastic spacer.
I forgot to mention that the Supreme version has screw in internal chokes as well, whereas the regular version is IC & M fixed only.
Rick