I believe the Lyman 45 sizer uses the same top punches as the 450. It's the model that was modified to become the 450.
Try checking the hole and the setscrew that secures the top punch. Sometimes the hole edges get burred by the setscrew and prevents it from going in.
Or, compare the stem of the top punch (if any) that was installed with the one that you are trying to fit. They should be the same diameter.