Clad quarters are notorious for filled mint marks. This especially applies to pre-'88 issues and more especially to 1981 (and '82).
They've made an awful lot of all the clads with worn dies. The mintmark was enlarged in '85 and this kept it from filling in as readily. The P mark was the major problem but filled D's were not too rare either. In '88 the relief was lowered enough that design fill was much better and the sharper mint mark just didn't fill as much.
