After a tablet was written, and sometimes sealed, the scribe would often take a piece of clay and flatten it against a level surface like a piecrust. Then he would fold the document into the sheet of clay. All excess clay would be pinched off, leaving enough to cover the document. On this he would write the same transaction in identical terms. The witnesses would affix their seals to the envelope as well.
The envelope was tamper-resistant: the contents inside could be revealed in any kind of dispute. The scribe had a special way of affixing the envelope to the document so that it didn’t stick: the envelope would be cracked off the document as a shell would be removed from a walnut.