Thank you for all your replies, although much of the information has gone right over my head.
Last night I fitted one of the lenses from the Pentax 35mm SLR film camera onto my Pentax *ist DL camera and, even though I had the aperture ring set on "A", and my camera on auto-focus, the view-finder indicated "MF" and the auto-focus function would not work.
I have attached an image of the lens mount.
I managed to take a picture in manual focus mode though.
Any advice or suggestions to get the auto-focus to work?