Difference between מלכו and מלכותו

Question

I was wondering what the difference in meaning (and thus, in the interpretation of Pesukim in the Megillah, as well as various Pesukim in Tanach) between למלכו and למלכותו, specifically according to Rashbam (Esther 1:3), who says:

למלכו – למלך הוא

I will add an answer from another source, but I would like to know if someone can find other explanations, especially one that would explain this Rashbam, or ones that help me understand the Dikduk of these two words.

To be clear, I am looking for this to help me interpret Esther 1:3 and 2:17.

Edit: Some other occurrences of each word:

למלכו:

וַיֵּצֵא יְהוֹיָכִין מֶלֶךְ יְהוּדָה עַל מֶלֶךְ בָּבֶל הוּא וְאִמּוֹ וַעֲבָדָיו וְשָׂרָיו וְסָרִיסָיו וַיִּקַּח אֹתוֹ מֶלֶךְ בָּבֶל בִּשְׁנַת שְׁמֹנֶה לְמׇלְכוֹ (Melachim B 24:12)

בִּשְׁנַת אַחַת לְמׇלְכוֹ אֲנִי דָּנִיֵּאל בִּינֹתִי בַּסְּפָרִים מִסְפַּר הַשָּׁנִים אֲשֶׁר הָיָה דְבַר י"י אֶל יִרְמְיָה הַנָּבִיא לְמַלֹּאות לְחׇרְבוֹת יְרוּשָׁלַ‍ִם שִׁבְעִים שָׁנָה (Daniel 9:2)

וַיְהִי בַשָּׁנָה הַתְּשִׁעִית לְמׇלְכוֹ בַּחֹדֶשׁ הָעֲשִׂירִי בֶּעָשׂוֹר לַחֹדֶשׁ בָּא נְבוּכַדְרֶאצַּר מֶלֶךְ בָּבֶל הוּא וְכׇל חֵילוֹ עַל יְרוּשָׁלַ‍ִם וַיַּחֲנוּ עָלֶיהָ וַיִּבְנוּ עָלֶיהָ דָּיֵק סָבִיב (Yirrmiyahu 52:4)

למלכותו: (actually, only in Esther and Divrei Hayamim B, I think)

וַ֠יָּ֠חֶל לִבְנ֞וֹת בַּחֹ֤דֶשׁ הַשֵּׁנִי֙ בַּשֵּׁנִ֔י בִּשְׁנַ֥ת אַרְבַּ֖ע לְמַלְכוּתֽוֹ (Divrei Hayamim B 3:2)

וַיֶּחֱלֶא אָסָא בִּשְׁנַת שְׁלוֹשִׁים וָתֵשַׁע לְמַלְכוּתוֹ בְּרַגְלָיו עַד לְמַעְלָה חׇלְיוֹ וְגַם בְּחׇלְיוֹ לֹא דָרַשׁ אֶת י"י כִּי בָּרֹפְאִים (Divrei Hayamim B 16:12, note that in the next Passuk, about his death, it uses Lemolcho)

Answer 1

Rav Eliezer Ashkenazi in his commentary to Esther, Yosef Lekach (2:17, see 1:3 as well), explains that Achashverosh originally was only the King of Persia, and he later took control of all 127 provinces. The word למלכו refers to counting from the his taking control of all 127 provinces, whereas the word למלכותו refers to from the date of his becoming King of Persia.

However, this would not explain the various other places in Tanach where both words are used (a search of Sefaria or Alhatorah yields many results for both words), and no similar situation has taken place with that king.

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Edited by DanF:

I asked my shul rav about the difference. He says that there is a slight nuance in meaning with the two words.

מלכו means "his rulership" or "his reign". מלכותו means "his kingdom". Thus, while Achashverosh was the ruler of 127 provinces, it's possible that he may have appointed others to manage those areas. His kingdom was in Persia.

Answer in progress regarding the new verses that you cited. I am concentrating only on the 1st batch למלכו:

Verse 1 - See Rada"k, esp, the end of it. He posits that there is an inconsistency regarding the time that Yohyachin began ruling. But, of interest is the end that says that says that the 18 years refers to the 18th year of Nebuchadneztar's kingdom. Thus, it's possible that he was just a ruler, while not having any title as king.