The Post {front page, Aug. 21} quotes Charles Merrill Mount as saying that materials he allegedly took from the National Archives ''had been in my possession for 25 years.''

I had some experience with the Archives many years ago that could account for how the materials came into Mr. Mount's possession. The situation has always amazed me, and for 30 years I have been wanting to tell this story to someone.

In 1957 I was on a government detail in Washington for several months. I had a personal interest in some aspects of Gen. George Crook's career, and during the day I would call the Archives, describe the material I was interested in seeing and then go there in the evening. I would go into a large room filled with tables at which others were working and go to a desk to pick up a large accordion envelope with the materials I had requested. As far as I could determine, the man at the desk was the only employee on duty in the room, though it is possible there was also someone else keeping an eye on things.

I would then dump the contents of the envelope on the table, many of which were in Gen. Crook's own hand and almost all of which were one-of-a-kind originals. I could, if I wished, place my own briefcase on the table and take out my note paper and other items, and I could easily have mixed my own materials with those of the Archives. There were many documents there I would have loved to have had, and I remember that there was at least one printed item of which there was more than one copy. What harm would there have been in mixing one of the copies in with my own materials? I could have placed it in my briefcase, placed the remaining material back in its envelope and returned the envelope to the desk.

By now, and I assume for a long time, any remaining documents have been placed on microfilm and users work with that.

FRANK CASANOVA

Carefree, Ariz.