How does Blomqvist know which safe deposit box to open?
He gets a bunch of Lisbeth's keys from her hospitalised friend and one looks like a key to a postal box or safety deposit box. But how does he know where it is, or what number box it is?
shareHe is an investigative reporter. He investigates. He finds.
The movie has 120 minutes to tell the whole story in the book.
Do you seriously want to waste those minutes watching the process of how he found a PO Box?
Just accept that he finds it. Don't pick at it.
/Keys don't have numbers on them, but they're all the same. You go to every freaking post office in the area and try every freaking box until you find one that opens, and then you hope it was the one you wanted to find, because they only have a few dozen key patterns, so just finding a box that opens might not be the box you want. And you keep checking boxes until you find the one you need, or until you find something else that renders that PO Box irrelevant, or you stop looking for the box because it doesn't matter anymore.