Probably nothing. The advantage of the UnLatch series is that it replaces the standard ANSI 4-7/8" strike plate without any cutting or modification. If you already have the door prepared for a 4-7/8" strike plate you are ready to go. Just verify that there is sufficient space in the jam to accept the body of the lock. The UnLatch requires 3-3/8" Long, 1-3/8" Wide, 2" Deep while the Mortise UnLatch requires 3-3/8" Long, 1-7/8" Wide, 2" Deep.