This is the Hebrew letter "mem מ.” It is spelled with open mem מ, and closed mem ם. It is one of five letters in the Hebrew alphabet that takes a final (sofit) form. The letter is written מ when occurring at the beginning or inside a word, but it is written ם (mem sofit ) when it occurs as the last letter of a word. This is sometimes called the open mem מ, because of the small opening at the lower left of the letter. In words where it is the final letter, it is written as a closed mem ם with no opening.
Yeshayah 7:14 Therefore God Himself shall give you an ot (sign); Hinei, HaAl
mah (the young woman, virgin) shall conceive, and bear a son, and shall call Shmo Immanu El
Almah from a root implying the vigour of puberty, is a Hebrew word for a
young woman of childbearing age
Which is important because ever other miraculous birth was by a barren woman beyond child barring age also represented by a closed mem ם
The term “l'marbeh l'marbeh ל
םרבה למרבה” literally “to the increase” denotes a large increase; a distinction that is lost in our English translations. But there are many other connotations here, one of which is explained by the sages as moshiach משיח will first come closed, in secret, then moshiach משיח will come openly. A closed mem is a closed womb; an open mem is the womb opening to give birth (Bahir 84). The sages said that the first time moshiach משיח comes, he will come in closed mem ם, a description of the coming of the moshiach משיח from a closed womb; from a virgin!
Isaiah 9:6
Of the increase
(“l'marbeh l'marbeh ל
ם
רבה למרבה”)
of his misrah and shalom there shall be no ketz (end), upon the Kisse Dovid, and upon his kingdom, to strengthen it, and to support it with mishpat and with tzedakah from henceforth even ad olam (forever). The Kinat Hashem Tzva'os will fulfill this.
So Isaiah 9:6 is telling us that the son in Isaiah 7:14 is the Messiah! This is why the Ruach HaChodesh inspired Yeshayahu to break the rules of Hebrew grammar and write “l'marbeh l'marbeh ל
םרבה למרבה.” The lamed ל prefix means "to" or "for." The first letter of the word is them men sofit ם, the closed mem; I.e. the closed womb of a virgin.
Of course contemporary Rabbis do not see HaMoshiach Yeshua in Isaiah’s prophecies, but they are aware of the closed mem in “l'marbeh l'marbeh ל
םרבה למרבה” and its Messianic portent.
“The rabbis teach that when it is time for the Redemption, the closed mem of Isaiah’s l’marbeh will open for the coming of the Messiah” (Radak, Isa. 9:6). Radak is Rabbi David Kimchi, a noted grammarian and scholar. Rabbi Kimchi got the literal meaning correct, but he has missed the true meaning. The child spoken of in this prophecy would be born of the closed womb of a virgin.
You've probably heard it said that Moshe (Moses) was a shadow of Messiah. There's more revealed in the Hebrew grammar like connecting His sister Miriam to Yeshua's mother, to Rachel and also to Jacobs wrestling with the angel of the Lord to Yeshua. All in the Masoretic text not the LXX.
And so it happened, the young virgin Miriam מִרְיָם, whose name begins with mem מִ and ends with men sofit ם, gave us a child, Yeshua HaMoshiach (Jesus the Messiah).
So just because the Early Greek Church commentators didn't understand this and challenged the translation doesn't mean its wrong. It means they didn't understand and they weren't looking at the whole context.