G'Day Fella's,

Nitro-X, I just went for a look about, and from what I can work out, Sig have used some marketing/political speak, to give you the impression that the frame is Stainless Steel.
Further to this, Sig suggest that the "Frame is Serialised" (has a Serial Number).
This is is correct but it appears that Sig consider the Fire Control System (which is made from Stainless Steel), to be the "Frame" and it has the firearms Serial Number on it.

Given the pistol has a series of interchangeable "Grips" of different size (Large, Medium, Small. And from what I can see, made from some synthetic material but it appears, Sig deliberately make no mention of this!) to match the shooters hand (very innovative!), it would be difficult to do this, if each different sized grip had to have a serial number. So I hope I am correct with this and that this possibly answers your question John.

FYI, here's a link; https://www.sigsauer.com/edu/meet-the-p320/

Doh!
Homer