Like the other four sofit letters, Final Mem's distinct shape is credited in the Talmud, at Shabbat 104a, either to the early prophets or to the men of the Great Assembly, a debate the text leaves open rather than settling in favour of either tradition. Final Mem's own shape is a fully closed square, in contrast to the open-topped form of ordinary Mem used everywhere else in a word, discussed elsewhere in this atlas; later Kabbalistic writers took that closure as an image in its own right, speaking of a mem setumah, a sealed mem, as a figure for teachings or future matters held back from open view until their proper time. That reading sits naturally beside Mem's ordinary meaning, water, and its place among the Sefer Yetzirah's three mother letters, discussed elsewhere in this atlas, since a closed vessel of water offered ancient commentators a ready image for something contained rather than flowing freely.
The specific, unrelated anomaly of a closed-style Mem appearing mid-word in Isaiah 9:6, recorded in the Talmud at Sanhedrin 94a, belongs to the history of ordinary Mem rather than to Final Mem's own separate development. In the extended gematria system known as mispar gadol, discussed elsewhere in this atlas, Final Mem carries the value 600, one of five values that extend the ordinary twenty-two-letter numeral count past 400.