[garethrees]

The purpose of the letter lock is to detect tampering: that is, to make it difficult for an eavesdropper to open and read the letter without leaving evidence of having done so.

The standard letter lock was a wax seal: that is a blob of sealing wax with an impression of a stamp or signet ring. A signet ring had an intricate design that was difficult to forge.

A standard wax seal could be defeated by sliding a thin heated knife between the seal and the paper. This melted a little of the wax at the base of the seal, enough to separate it from the paper. Then the letter could be read and the wax seal reattached using the same heated knife.

The purpose of the complex letter locks described in the article was to make the hot knife technique more difficult, by embedding the end of a cut-out strip of paper into the seal so that when the eavesdropper slid their hot knife underneath the seal it would cut the strip.