I have a 70's Security Six as well with the typical brushed stainless finish. I'll let someone more knowledgeable comment about whether or not the factory ever produced a polished stainless Security Six, but I can comment on the finish.
It's actually quite easy to put a mirror polish on a brushed stainless firearm. All it takes is some Mother's Mag polish, an old white cotton t-shirt, elbow grease and patience. To facilitate getting into crevices, I made some oak dowel tools (round, point & chisel) to push the t-shirt into these areas. Very minor marks and scratches can be removed with Scotch Brite pads. I typically use the grey pads for scratches, then follow up with the white. The maroon pads are much more aggressive, but are great for touching up the flats of the Mark II Competition model. Don't use them on a brushed stainless finish or you'll have more scratches than you'll know what to do with.
This is probably more than you wanted or needed to know, but my point in this is that if I were a betting man, my money would be on that polished finish being post-factory.
Hopefully I'm wrong and you have an unusual revolver.