Michael R. Freeman , Aug 12, 2012; 10:51 p.m.
The PK rings are Ai compatible, ie. meter coupled and auto-diaphragm.
The K rings are not meter coupled (stopdown metering is required), are not auto-diaphragm (finder dims as you turn the aperture ring) and are stacked together for various extensions. Some of the K rings can not be used on modern bodies.
The K1 ring can be used on most modern bodies, and is an excellent extension ring for up close work with manual focus wide angle lenses. It's the thinnest F-mount extension ring available (5.8mm).