In Parshas Mishpatim, the Torah tells us of the laws regarding an Eved Nirtzah - A Jewish slave who has finished his term of slavery yet chooses to stay as a slave by his master, as a consequence we bore a hole in his ear against a doorpost and he then remains a slave by his master until Yovel.
The written text in the parashah reads as follows:
וְהִגִּישׁ֤וֹ אֲדֹנָיו֙ אֶל־הָ֣אֱלֹהִ֔ים וְהִגִּישׁוֹ֙ אֶל־הַדֶּ֔לֶת א֖וֹ אֶל־הַמְּזוּזָ֑ה וְרָצַ֨ע אֲדֹנָ֤יו אֶת־אׇזְנוֹ֙ בַּמַּרְצֵ֔עַ וַעֲבָד֖וֹ לְעֹלָֽם׃
his master shall take him before God. He shall be brought to the door or the doorpost, and his master shall pierce his ear with an awl, and he shall then remain his slave for life.
The literal reading of the text would imply that one has the option of boring the hole either on the door or on the doorpost, yet Chazal (Kiddushin 22b) clarify that the doorpost is in fact an invalid place to bore the hole, as the Torah itself states (Re'eh, Devarim, 17;15)
וְלָקַחְתָּ֣ אֶת־הַמַּרְצֵ֗עַ וְנָתַתָּ֤ה בְאׇזְנוֹ֙ וּבַדֶּ֔לֶת וְהָיָ֥ה לְךָ֖ עֶ֣בֶד עוֹלָ֑ם וְאַ֥ף לַאֲמָתְךָ֖ תַּעֲשֶׂה־כֵּֽן׃
you shall take an awl and put it through his ear into the door, and he shall become your slave in perpetuity. Do the same with your female slave.
We must specifically bore the hole on the door alone, the reason that the Torah mentions the doorpost in the verse prior is to teach us that the characteristics of the door must be similar to that of the doorpost, just as the doorpost is perpendicular so too the door must be perpendicular and on its hinges when performing the piercing.
In Aderes Elyahu, the Vilna Gaon comments on that verse:
פשטא דקרא גם המזוזה כשרה אבל הלכה עוקרת את המקרא וכן ברובה של פרשה זו וכן בכמה פרשיות שבתורה והן מגדולת תורתינו שבע"פ שהיא הל"מ והיא מתהפכת כחומר חותם חוץ המצות שבאו במנצפ"ך שהם מישרים
The simple reading of the verse implies that one can bore a hole in the ear of the Jewish slave on the doorpost, yet the oral law uproots the textual reading of the verse, This is true for most of this parshah as well as many other parshiyos in the Torah. This is due to the greatness of the Oral Torah which was transmitted by Moshe from Sinai, as it reverses like the mold of a stamp, except for the commandments that come from Mantzepach which are straight.
The Gaon is teaching us that the oral Torah and the written Torah are one and the same, the only difference is perspective. The wonderful analogy he uses to describe this phenomenon is the mold of a stamp in respect to its imprint on paper. The mold of the stamp has the lettering backward and is only spelled properly when stamped on paper, so too the written Torah is to us like the mold of the stamp whereas the oral Torah is its imprint. A tremendous parable that enlightens the mind to the relationship of the oral Torah and the written Torah.
There Vilna Gaon writes that there is an exception, however, as the mitzvos from Mantzepach are straightforward. We know that Mantzepach is an anachronym for the end letters in the Hebrew alphabet as seen from the Talmud (Megillah, 2b-3a). But what exactly are the mitzvos that are learned from Mantzepach? Furthermore, in what way are these mitzvos straightforward as opposed to the other mitzvos?
Thank you for all suggestions in advance!