SuttaCentral
In the Book of Analysis, the chapter dealing with Path Constituents, the section of Interrogation by Triplets, SuttaCentral , it is said;
[The Eight Path Constituents] Sometimes are of the seven supramundane stages; sometimes are of the final supramundane stage.
What are the Supramundane Stages therein?
I believe it refers to the four stages of awakening, each divided into path and fruit, making eight. As in AN 8.59 and other suttas using the same model.
This was also the only explaination i had if the total of stages are Eight;
Faith & Dhamma-Followers; are the 1st out of 4 types of learners with Path; are without fruition; are striving for the 1st Fruition [putting away the three fetters];
Stream-Attainer; is the 2nd type out of 4 types of learners with Path; is the 1st type of 3 learners with Fruition; has the 1st Fruition [put away the three fetters]; is working for the 2nd Fruition [reduction of attachment sensual pleasures and malevolence to residue]
Once-Returner; is the 3rd type of learner out of 4 with Path; is the 2nd type of 3 learners with Fruition; has the 2nd Fruition [reduced attachment sensual pleasures and malevolence to residue]; works for 3rd Fruition [putting away the attachment to sensual pleasures and malevolence without a residuum);
Non-Returner; is the 4th type of learner out of 4 with Path; is the 3rd type of out of 3 learners with Fruition; has the 3rd Fruition [put away the attachment to sensual pleasures and malevolence without a residuum]; works for 4th Fruition [putting away attachment to form and the formless, to pride, haughtiness, and ignorance without any residuum]
Arahant; Is neither a learner nor a non-learner; has the 4th fruition [put away attachment to form and the formless, to pride, haughtiness, and ignorance without any residuum]
There a Sotapanna is both 2nd of those learners with Path and 1st of those learners with fruition.
As the interrogation is worded it was somewhat confusing but i think you are correct as i can’t see another explaination. It does seem strange to me tho, in terms of the Individual types it is clear and obvious how Individuals would be thus classified by the 8-fold classification but in terms of Stages it is not as clear & obvious to me. As i discern it, it doesn’t easily transform to Individual classification as above;
Path Constituents in the 1st Stage - Ie Right Speech in one developing 1st Path for the attainment of 1st Fruition [putting away the three fetters]
Path Constituents in the 2nd Stage - Ie Right Speech in one with 1st Fruition; developing 2nd Path for the attainment of 2nd Fruition
Path Constituents in the 3rd Stage - Ie Right Speech in one with 2nd Fruition; developing 3rd Path for the attainment of 3rd Fruition
Path Constituents in 4th Stage - Ie Right Speech in one with 3rd Fruition; developing the 4th Path for the attainment of 4th Fruition
Path Constituents in 5th Stage - Ie Right Speech in one with 4th Fruition
Therefore i find it difficult to see how the Stages are to be classified
Like @Christopher stated, it most likely refers to the 4 stages of enlightenment divided into path and fruit, which equals 7 stages plus the “final” one of arahantship. An article supporting this idea… (see near the end of the essay).
Edit: Look for the sentence “Thus the supramundane should be understood as eightfold,” and read from there.
What also throws me off is if one differentiates between the 8-Fold Path and 5-Fold Path (consciousness without speech/action/livelihood) as well as the path factors associated and not associated with Release, this adds some variables, therefore i think maybe there is an alternate explanation 
This explanation is a combination of the eight stages of the discourses, and the seven stages of the Vism., based on insight. (Vism. XXI, 74)
It’s worth bearing in mind that these stages are conceptualized quite differently in the suttas and the commentaries.
In the suttas, these are kinds of people who have attained various stages along the path.
In the commentaries, they are mind-moments, flashes of realization lasting an ultra-nano-second.
The canonical Abhidhamma in Pali is an intermediate stage and could possibly be read both ways; but there was no explicit theory of mind moments in the canonical period.
Bhante, I’m not sure how short an ultra-nano-second is… 
But are now laser pulses measured in attoseconds…
I agree that momentariness, along with dependent origination and anatta in isolation, should be avoided by over-ambitious beginners, imagining they can scale the heights before doing the groundwork. Referring to the Visuddhimagga only, it is a meditation manual and teaches in a graduated fashion. On a general reading it might appear to be overwhelmingly concerned with momentariness, but on closer inspection is seen to recommend a gradual development of knowledge of impermanence beginning with comprehension of materiality before mentality. This in itself avoids much of the focus on momentariness. The exercises on materiality start with a foundation in impermanence in natural phenomena, the human life cycle over a hundred years, the seasons, the phases of the moon over one month, the stages of the day. These exercises in knowledge of impermanence should be consolidated before any attempt at comprehension of mentality can be made, because that (materiality) is the source of the consciousness that arises. In my view, lay practitioners could spend a lifetime of practice profitably establishing the foundation of the comprehension of materiality. Not to base the practice in reality, which is a danger facing online western Theravada, denies the characteristic of suffering.
I am now thinking that i would explain it as;
Fourfold rapt meditation developments of the Paths and the absorbtion resultant of that very development which is comprehended as Signless/Empty/Undirected which is on the treshold of Extinguishment [the cessation of aggregates] and is here a destruction of the taints.
So the final stage would not be the state of the Arahant, one with purified intellect. The final stage would be the final stage of the removal of delusion, having attained which one emerges an Arahant.
I also don’t think the attainment of this absorbtion on the treshold of extinguishment is necessarily a nano-second ordeal but a state of highest pleasure where nothing is felt and is a discernment of the Unmade, lasting for a pre-determined amount of time and/or up to seven days depending on prior development; is the type of absorbtion not dependent on earth, water … nor dimension of neither-perception-nor-non-perception.
That’s how i understand it.
It is true that there’s nothing stated, explicitly.
Yet how quickly can one aggregate arise before it is replaced by another? Neurological research shows the brain responding from 10 milliseconds (ms) to subsequent reactions up to around 200 ms, later. Visually presented impulses would correspond with Rupa and a volitional reaction would correspond to cetana. Time for auditory and visual stimuli to reach - Mammals - BNID 110801
These are my experiences:
