Dav Yomi: What Was The Mark?

I posted last Friday about the instances of Ktav Ivri {Paleo-Hebrew script} in the gemara and Tanach. One I mentioned was a Yerushalmi which spoke of the ayin and tes on the luchot in which the centers suspended miraculously, as opposed to the Bavli which has samach and mem sofit, the difference being that one refers to Ktav Ashurit and one refers to Ktav Ivri.

I also referred to the psukim in Yechezkel of making a mark (tav) on people's foreheads. It turns out this pasuk is mentioned on today's daf (Shabbat 55a).

ד וַיֹּאמֶר ה, אֵלָו, עֲבֹר בְּתוֹךְ הָעִיר, בְּתוֹךְ יְרוּשָׁלִָם; וְהִתְוִיתָ תָּו עַל-מִצְחוֹת הָאֲנָשִׁים, הַנֶּאֱנָחִים וְהַנֶּאֱנָקִים, עַל כָּל-הַתּוֹעֵבוֹת, הַנַּעֲשׂוֹת בְּתוֹכָהּ.	4 And the LORD said unto him: 'Go through the midst of the city, through the midst of Jerusalem, and set a mark upon the foreheads of the men that sigh and that cry for all the abominations that are done in the midst thereof.'
ה וּלְאֵלֶּה אָמַר בְּאָזְנַי, עִבְרוּ בָעִיר אַחֲרָיו וְהַכּוּ: על- (אַל-) תָּחֹס עיניכם (עֵינְכֶם), וְאַל-תַּחְמֹלוּ.	5 And to the others He said in my hearing: 'Go ye through the city after him, and smite; let not your eye spare, neither have ye pity;
ו זָקֵן בָּחוּר וּבְתוּלָה וְטַף וְנָשִׁים תַּהַרְגוּ לְמַשְׁחִית, וְעַל-כָּל-אִישׁ אֲשֶׁר-עָלָיו הַתָּו אַל-תִּגַּשׁוּ, וּמִמִּקְדָּשִׁי, תָּחֵלּוּ; וַיָּחֵלּוּ בָּאֲנָשִׁים הַזְּקֵנִים, אֲשֶׁר לִפְנֵי הַבָּיִת.	6 slay utterly the old man, the young man and the maiden, and little children and women; but come not near any man upon whom is the mark; and begin at My sanctuary.' Then they began at the elders that were before the house.

Now, one can take this as a random mark, but perhaps one should take it as a mark inspired by the letter which bears the same name. In Ktav Ashuri this looks like ת, which is strange for a mark. But in Ktav Ivri it looks like an X.

On the daf, they seem to understand it as referring to the letter, making statements like "from aleph to tav," though this might be dismissed as concrete proof since they are after all making drashot, and a drasha can rely on such a "homonym."

But, this transforms it from being just the pesukim in Yechezkel as that which one needs to know Ktav Ivri to understand, into the gemara in Shabbat 55a.