I didn’t take them as being proto-abhidhammic (I thought that was Sāriputta…), they seemed quite straightforward. Here is a selection from the Bodhi/Nanamoli translation of the Majjhima Nikaya:
“Dependent on the eye and forms, eye-consciousness arises. The meeting of the three is contact. With contact as condition there is feeling. What one feels, that one perceives. What one perceives, that one thinks about. What one thinks about, that one mentally proliferates. With what one has mentally proliferated as the source, perceptions and notions [born of] mental proliferation beset a man with respect to past, future, and present forms cognizable through the eye.
“It is just a saying in the world, great king, that ‘Brahmins are the highest caste…heirs of Brahmā.’ And there is a way whereby it can be understood how that statement of the brahmins is just a saying in the world.
“What do you think, great king? If a noble prospers in wealth, grain, silver, or gold, will there be nobles who rise before him and retire after him, who are eager to serve him, who seek to please him and speak sweetly to him, and will there also be brahmins, merchants, and workers who do likewise?”
“How, friends, does one revive the past? One’s consciousness becomes bound up with desire and lust there thinking, ‘My eye was thus in the past and forms were thus.’ Because one’s consciousness is bound up with desire and lust, one delights in that. When one delights in that, one revives the past.
“One’s consciousness becomes bound up with desire and lust there thinking, ‘My ear was thus in the past and sounds were thus…My nose and odours…My tongue and flavours…My body and tangibles…My mind was thus in the past and mind-objects were thus.’ Because one’s consciousness is bound up with desire and lust, one delights in that. When one delights in that, one revives the past. That is how one revives the past.
“How, friends, is consciousness called ‘distracted and scattered externally’? Here, when a bhikkhu has seen a form with the eye, if his consciousness follows after the sign of form, is tied and shackled by gratification in the sign of form, is fettered by the fetter of gratification in the sign of form, then his consciousness is called ‘distracted and scattered externally.’
“When he has heard a sound with the ear…smelt an odour with the nose…tasted a flavour with the tongue…touched a tangible with the body…cognized a mind-object with the mind, if his consciousness follows after the sign of the mind-object, is tied and shackled by gratification in the sign of the mind-object, is fettered by the fetter of gratification in the sign of the mind-object, then his consciousness is called ‘distracted and scattered externally.’
I’m not sure if it is significant that these discourses use the sense-base analysis of experience, rather than the aggregrate analysis. This is particularly obvious in MN 133, which comments on MN 131, which analyses according to the aggregates.
And, of course there is the endearing pericope when the other bhikkhus ask him to expand the meaning:
[The venerable Mahā Kaccāna replied:] “Friends, it is as though a man needing heartwood, seeking heartwood, wandering in search of heartwood, thought that heartwood should be sought for among the branches and leaves of a great tree standing possessed of heartwood, after he had passed over the root and the trunk. And so it is with you, venerable sirs, that you think that I should be asked about the meaning of this, after you passed the Blessed One by when you were face to face with the Teacher. For knowing, the Blessed One knows; seeing, he sees; he is vision, he is knowledge, he is the Dhamma, he is the holy one; he is the sayer, the proclaimer, the elucidator of meaning, the giver of the Deathless, the lord of the Dhamma, the Tathāgata. That was the time when you should have asked the Blessed One the meaning. As he told you, so you should have remembered it.”