I had one that bought in Nov 2008 and it had the same mag tube as yours (black with brass ring).
I now have another one that Winchester/Browning sent me a few days ago and the mag tube is stainless steel.
I'm beginning to wonder that mine is an older model because if they made it more light weight, the stainless mag tube has a couple of places on it that are thicker causing more weight.
Also the gas piston has to be larger to go around the thick part of the mag tube causing more weight.
Then the barrel where the gas ports are, has to be bigger to allow the larger gas piston to fit into it causing more weight.
This is what mine looks like (cell phone pic). Notice the thick part closer to the top where the gas piston sits.