The bolt locking extension rides in a slot in the receiver and engages the bolts "safety lug", therefore locking bolt shut on safe. On my newer .375 I had to cut the slot in the receiver to accept the safety extension. The older receivers already have this slot. The easier way to have a bolt locking safety on the newer Zastavas is to install a W70 style 3 position safety like Ontario Hunter did which is great if you like that style safety. I do not, so modified my rifle's receiver to work with the bolt locking safety. I also added a raised nub to the thumbpiece of the safety to make safety manipulation easier and more reliable in high stress situations. Pictured is the slot I cut in receiver, and the raised nub I added to thumbpiece.