What kind of lens board / how thick? Are you sure there is no anti-rotation pin sticking out of the back of the shutter? A Copal shutter will often fit on a fairly thin lens board, but sometimes there is a little pin on the back that prevents rotation by fitting into a hole in the board. If your board doesn't have a matching hole, then the shutter doesn't fit square to the board and can't be screwed down. If this is the issue, you can usually remove the pin with a small screwdriver, or drill a matching hole in the board.