Is the contents of a Mann Co. Supply Crate set before it is opened?

The only items I can market are Mann Co. Supply Crates. And, if I have a look at the description of the crate, I can see a list of possible items:

Does this mean that if I try to open the crate a pseudo-random number would be drawn and one item would be given to me? Or is the item already set when I first got the crate? This is especially important for the "Exceedingly Rare Special Item"!

Strangely, if the draw had yet to happen, then I would have no problem selling the crate. But if the draw already happened, then I would like to know the result, and I would be a bit reluctant to sell the crate. It is all about psychology, because I understand there is no difference in fact.