What is the secret behind the masculine form of the word "butun" (بطون) in verse 66 of Surat An-Nahl and its feminine form in verse 69, and what is the source that clarifies these subtleties?

The pronoun for An'am (livestock) in His Almighty's saying: (مِمَّا فِي بُطُونِهِ) "from that which is within its bellies" is masculine, observing the singular form of the word, as An'am is a collective noun singular in its wording. Whereas the pronoun in the verse from Surat Al-Mu'minun: (نُسْقِيكُمْ مِمَّا فِي بُطُونِها) "We give you to drink from that which is in their bellies" is feminine, observing the collective meaning of An'am. As for An-Nahl (bees), it is a generic noun that can be masculine or feminine. The feminine usage is the language of the people of Hijaz, and the masculine usage is the language of others.