Looks like one part of the clasp should be ...

Usually on these types of clasps, there is a little metal tang with a notch on the inside of the part that the worded end of the clasp slips over a bar. Under that bar is where you lift up the notched tang. After you lift it up, using a miniscrewdriver or watch knife, you can then slide the attachment to the desired band length. Then push the tang back down to lock it.