Ok, we've been getting some champagne as gifts and we got a strange one today. It had a huge steel clamp on the top of it as well as a shrink wrap. It is a metal clamp on top and the cork itself is really compressed like 2-3mm so that you can't just slide it off.
Turns out we have not ever seen this but some googling led us to the Lambrusco article. Turns out that you can take a butter knife and slide it between the clamp and the cork.
Despite the appearance that the clamp is under pressure, in reality it's just a safety device and there is no pressure on the lower part. So it is mild aluminum I would guess and you can just pry it open.
Be careful as the cork could launch but in our case the pressure on the cork kept it in. And as always, you should the champagne at a 45 degree angle and then twist the bottle and have a cloth over the cork.
And it will come right out.
The Gray Report: Overwhelmed by trying to open a bottle of Lambrusco
— Read on blog.wblakegray.com/2014/07/overwhelmed-by-trying-to-open-bottle-of.html