Yes, matte stainless steel is actually the most common of the low-end Sonnets. I have seen the plain steel ones too, but not as often.
The Matte Stainless Steel is often simply called Stainless Steel for convenience. In fact, if you goto Parker website (
here) and scroll to the far right, you will see that only the Steel Matte Sonnets are displayed, and not the one in the plain polished steel.
Upon googling, here is the common Steel Matte Sonnet (with Gold finishes and GP nib)

Unfortunately, I don't seem to be able to find a picture of the plain steel Sonnets (not matte) - they're not as common, and have probably been replaced by the Matte Sonnets. I have seen it before, with my two eyes at a Brick n Mortar store, clearly on sale as a "Stainless Steel" Sonnet at a price of roughly $90~100-ish.