Richard,
I have not cracked open my Sunday Times yet, but the aluminum SL was introduced in Detroit LAST year, at the [January] 2012 Detroit North American International Auto Show. You can see the photo here, of the cut-away chassis/body they had on display ONLY during the press/media days which precede the general show.
Here's the general format: Sunday starts the parties and special off-site reveals. Monday and Tuesday are the Media days, for accredited journalists only. Wednesday and Thursday are Industry days, when Engineers, etc. come by and see the offerings. Friday is the Charity Preview, and general show opening is on Saturday. So, there are ONLY supposed to be accredited journalists on that Monday--having been through the accreditation process numerous times, it is not a frivolous process.
Why do I tell you this? Because on that Monday, with the very new and exciting aluminum chassis/body of the SL on display, the Chinese and other Asian "journalists" were photographing every square inch in and out of that unit, to the point of overt annoyance. They swarmed it like flies on a dung heap. I think Mercedes figured out they were not "journalists" but Chinese engineers, and they removed the chassis/body by Tuesday. Luckily I got to see it on Monday.
The photo I show here is from the rear of the car looking towards the inside front firewall; the two large round holes are the bass speaker ports, for the Hamon-Kardon "Front-Bass" system. It was challenging to get a good overall shot of this by itself due to location and crowds. Interestingly, there were also no photos of the chassis/body in the press kit.
The entire new SL is a showcase of aluminum technology--manufacture, fabrication, assembly, etc. There are many different kinds of aluminum involved, such as sheet, stamped, cast and forged. There are rivets, welds, and glue. Steel is integrated where aluminum won't work--such as windshield frame. Yes, rust is no longer much of an issue but ask any aviator--corrosion can rear its ugly head too.
