A careful examination of the K&F example photos shows that the adapter has a chamferred corner on the tang end by the red dot and again on the one adjacent to the retention spring. It is that chamfer done rather than a square cut that differentiates most (all?) non-genuine adapters and which has been associated with the tendency to overrotate slightly and become stuck in the mount. (The square cut edge on the genuine solidly engages the body-side spring). As noted above, the reported quality is supposed to be good and the price is certainly quite reasonable.
My advice, as always, is to try and source the genuine adapter. The cold chill of realizatioin when one of the brand-X adapters will not rotate out is true panic. I have only had it happen once, but once was enough. Mine happened with a white box, Bower-branded adapter from Adorama. It took me about four hours of mindless fiddling before it finally slipped free. I have no idea what I did to get it loose. As has been mentioned on other posts on this site, last resort is to cut it out or remove the K-mount from the camera and pry the body release spring manually from underneath.
If you do buy the K&F:
- Remove the flat metal spring as first step before you even think of attaching it to a camera. (Set the spring and screw aside.)
- Carefully attach the adapter to a lens you might care less for than others, flat side to. If the thread starts to bind, don't attempt to force or work it past a rough spot, remove immediately. Your lens is not intended to act as a tap for cleaning badly cut threads. The adapter should screw on with little resistance and snug up with the adapter's flange tight against the lens flange.
- Remove the adapter from the lens and carefully insert it into the mount throat on the body, taking care to align the red dots.
- Rotate the adapter into the mount. You should be able to mount it using finger a finger nail in the index notch and finger pressure alone. Most (all?) brand-x adapters are a tighter fit than the genuine*, but overly tight is not a good thing. The tangs on the camera side may be stainless steel, but they should not be used to shave excess thickness off the plated brass adapter. Any shards released would not be a good think to have in your shutter or on the sensor. The adapter will not lock in place due to the retention spring being removed. Note that the face of the adapter should be slightly inset below the face of the K-mount flange or at very most, exactly flush with its face. Any protrusion will disallow infinity focus.
- Carefully screw your lens onto the mounted adapter, taking care to note body clearance for any projections (e.g. the Auto/Manual switch). If there is any contact with the body other than at the lens flange, stop and reverse the lens out of the adapter. That lens will not work with your camera. It should snug up gradually against the camera's mount flange and when firmly mounted, you should not be able to see any part of the bright adapter body through any gaps. If you can, there may be a problem with the lens not having a broad enough lens flange. There are work-around solutions on this site.
- Work the focus ring and aperture ring. Ideally, the lens should remain firmly in place in the mount despite the retention spring being removed. If not, some other means will have to be used to hold everything in place.
Don't be concerned if the focus index mark on the lens is not at the top. They almost never line up, not even on a Pentax camera.
Steve