[FotoPete]

The SMC Pentax-A 70-210/4 and similar CPC Phase II variant both have loosening issues with the barrel.

The Push Pull Barrel for the lens is a two piece design. The outer shell that contains the rubber grip is held to the focus group that is part of the focus/zoom. The inner shell or column contains the focal length markers on it. For some reason that seems endemic to this lens with age, the 4 screws+collar that hold the outer shell to the inner column gets loose. One would notice the outer barrel slipping or rotating past the focal length extremes.

To fix these, pull back gently on the bottom end of rubber grip. You'll see a metal stripe running across the bottom and eventually four screws 90 degrees apart. In your case, I assume the previous owner had retightened the screwed but with the outer shell in the wrong position. (I made that mistake a couple times myself.)

Just flip the rubber and loosen slightly, the four screws so that there is noticeable play between the outer barrel and the inner column. Mount the lens onto your camera and using the clouds in the sky as reference, align the outer shell with the infinity mark on the column and then retighten the four screws and you're all set.