I took mine apart recently and there was a very tiny C clip between the butterfly and the tube that it sits in. I needed a magnifier to see it and a tiny pick to remove it. I seem to remember the flap may have been spring loaded but I'd need to check the bag of bits to be sure. I am fairly sure there was only one clip but I'd look real hard before you do anything.
I wouldn't go hammering anything until you are certain there's no clip there. One of mine was seized in even after the clip was out and I damaged the pin getting it out.
It was a real pig to get out though and I'm sure will be equally bad to reassemble. My chrome plater broke one and messed the other one up so I have another pair on the way to me, hopefully I'll have better luck with these ones. Easier with two of you also.