I don't know who made the first ammunition for the .357 Sig but Sig doesn't make anything in .357 S&W Magnum, do they? So I think Archie had the right answer.
The .30 Luger was not developed from the 9mm Luger; it was the other way around.
Shoot low, sheriff. They're riding Shetlands!
Underneath the starry flag, civilize 'em with a Krag,
and return us to our own beloved homes!
Buy War Bonds.