Well, by looking at my 4120, it appears that the copper extends into the tubing itself. In fact, the SS is not even visible externally. Hopefully a picture is worth more than these words:
Also note that Lytron 6XXX copper rads have a microscopic layer of paint, which might lower their performance to that of the SS rads. But I would guess that superior fin bonding plays a greater role in the equalization, which one might guess from the difference in price between copper and SS rads.