Perhaps Internal āyatana in RūpakKhanda refers to mental impression of those āyatana, Rather than physical āyatana themselves.
For example, Humans holds eye, ear, tongue etc as dear to them and hold them close.
RūpakKhanda also has past and future āyatana mental impressions.
Let’s say a person become blind at some point in his life, If he is not arhant then at cuti-paṭisandhi he will grasp a bhava similar to PañcaUpādānakKhanda. He might have desire to have future sense faculties just like he had in past before becoming blind.
p.s. I am not sure if there is difference between āyatana and Saḷāyatana, Is there?
I am clear on differences between six Indriya and twelve āyatana. (Indirya becomes āyatana under influence of taṇhā/avijjā.)