It seems that the earlier W/NG's retained the flute on top and the bell mouth on the end of the tail pipe (mine has this) I don't know when they changed to the straight tail pipe. But would be a lot easier to make.
I'm quite sure that the original finish was matt chrome plate, as seen in the factory picture that Luis posted. The easiest and cheapest option these days is to buy a bare metal system and VHT aluminium spray paint. The more expensive option is a stainless steel system and mild grit blast to a matt finish. The ultimate is to get the system matt chrome plated if you can find a firm to do it properly, but this would usually mean a new system only as most plating firms won't touch a used system. Ron
If the Chrome is good quality and in good condition, it's still quite possible to blast it with medium grit to matt the chrome surface. I've done quite a few smaller items, levers and this new bright chrome 16H silencer. Best if it's good old fashioned British triple chrome and not the flash chrome as often used on the continent, Asia and USA Ron