Letter lock

a lock that can not be opened when fastened, unless certain movable lettered rings or disks forming a part of it are in such a position (indicated by a particular combination of the letters) as to permit the bolt to be withdrawn.

See also: Letter

Webster's Revised Unabridged Dictionary, published 1913 by G. & C. Merriam Co.
References in classic literature ?
It would have been impossible to carry a bad name with a greater sweetness of innocence, and by the time I had got back to Bly with him I remained merely bewildered--so far, that is, as I was not outraged-- by the sense of the horrible letter locked up in my room, in a drawer.
The Little Tramp star's link to the spot only emerged in 1991 after his daughter Victoria found a secret letter locked in a desk he had owned.
But in 1991 his daughter found a letter locked in a desk that suggested otherwise.