1989 is the most common answer. When you note RPMs on the sites, they all stop at that year. But the reason is that the mintmarks were hand punched into a hardened die. Thus the mintmark punches started out normal, but eventually they go disfigured. So why some look distorted.

That is why the locations were different. They were to be in a target area, but not an exact science. That ended in 1990 when the dies contained the mintmarks as part of the design. So since then, they are located in a certain location.
CoopHome :
Why are some mintmarks in different locations and when did they start being added by the design?