Master Keyed Wafer Lock
While traditional pin tumbler locks can be mastered fairly easily with additional pins, wafer tumbler locks can't utilize additional shearlines, and a more creative solution is required
Like the Sargent square pin above, this Timberline (now owned by CompX) uses a master & change key with different profiles, and stepped tumblers. Master key hits left side of wafer, change key hits right. Haworth, Steelcase, and other simple wafer locks use the same principle.
Since I picked this lock and housing up for $0.50, wasn't too keen on spending $10 for a master key. Modified the warding of a Y11, and cut the master off a picture .