PF 18mm F2.8 (Pan focus lens)

“The pan focus lens is always in focus. Its extensive depth of field lets you take pictures of any object from 1.75 meters away all the way out to infinity, without adjusting the focus. In fact, there’s no focusing ring to adjust. Pan focus pictures have a unique feeling of clarity extending far into the distance. The lens is great for sport and candid photos, too.” Pentax literature, June 1984


So why did Pentax produce a fixed focus lens? It came out very late in the production so my guess is that the flagging sales made Pentax think about realigning the market sector to include the much larger point-and-shoot sector. A fixed focus option enabled that goal. The rarity of these lenses now suggest that this move did not achieve the desired results of increased sales. 
This lens was available as a standard option with the body as seen below.