Here's Ven Nyanaponika's translation of the Alagaddūpama Sutta, with comments on the relevant section regarding what some regard as self. It certainly warns against various eternalistic notions that might arise if one is not vigilant about ruling out some rather subtle concepts of self.
15. “There are, monks, these six grounds for false views.  What are the six? There is here, monks, an uninstructed worldling who has no regard for Noble Ones, who is ignorant of their teaching and untrained in it; who has no regard for men of worth, who is ignorant of their teaching and untrained in it: he considers corporeality thus: ‘This is mine, this I am, this is my self’;  he considers feeling… perception… mental formations thus: ‘This is mine, this I am, this is my self’; and what is seen, heard, sensed, and thought;  what is encountered, sought, pursued in mind,  this also he considers thus: ‘This is mine, this I am, this is my self’; and also this ground for views (holding): ‘The universe is the Self.  That I shall be after death;  permanent, stable, eternal, immutable; eternally the same,  shall I abide in that very condition’—that (view), too, he considers thus: ‘This is mine, this I am, this is my self.’ 
18. [Grounds for false views (diṭṭhiṭṭhāna). Comy: By the words “There are, monks, these six grounds for false views,” the Master wishes to show this: “He who takes the five aggregates of existence as ‘I’ and ‘mine’, by way of a threefold wrong grasp (tividha-gāha), he flings mud and refuse into my dispensation, like this Ariṭṭha.”
Comy and Sub-Comy: False views themselves are “grounds” (or bases, starting-points) for subsequently arising false views, like personality belief, eternalism, etc. (Comy: diṭṭhi pi ditthiṭṭhānaṃ). Further, the “grounds” are the subject-matter (ārammaṇa, “object”) of the views, i.e., the five aggregates, the visual objects, etc. Finally, they are also the conditioning factors (paccaya) of the false views, e.g., ignorance, sense-impression (phassa), (faulty) perceptions and thoughts, unwisely directed attention (ayoniso manasikāra), bad company, others’ speech, etc. [These, with the aggregates as the first, are the eight “grounds for false views,” as mentioned in the Paṭisambhidāmagga (Diṭṭhi-kathā). The term diṭṭhiṭṭhāna also occurs in the Brahmajāla Sutta (DN 1) and in the commentary to it. [Back]
19. “He considers corporeality thus: ‘This is mine’.” Comy: This is wrong grasp (or wrong approach) induced by craving (taṇhā-gāha). ”This I am”: this is wrong grasp induced by conceit (māna-gāha). ”This is my self”: this is wrong grasp induced by false views (diṭṭhi-gāha). Here, reference is to craving, conceit, and false views which have corporeality as object; but corporeality cannot be said to be a self. The same holds true for feeling, perception and mental formations. [Back]
20. “What is seen”: (Comy) the visual sense-object base (rūpāyatana); “heard”: the sound-base; “sensed” (mutaṃ): the sense-object bases of smell, taste, and touch-sensations; “what is thought”: the remaining seven bases, i.e., the mind-object base (dhammāyatana) and the six sense-organ bases. [Back]
21. “Encountered”: (Comy) after having been sought for, or not sought for; “sought”: encountered or not encountered (before); “mentally pursued” (anuvicaritaṃ manasā): resorted to by consciousness (cittena anusañcaritaṃ)—what was encountered or not encountered without being sought for.
The terms “thought,” “encountered,” etc., refer to the fifth aggregate, i.e., consciousness (viññāṇakkhandha), which was not mentioned in the first part of §15. [Back]
22. “The universe is the Self,” lit.: “This (is) the world, this (is) the self” (so loko so attā). That, in fact, an identification of the two terms is intended here, will be shown in the following comments. The best explanation of the passage is furnished in the Brahmajāla Sutta (DN 1) where a similar phraseology is used: “There are, monks, some ascetics and brahmans who are eternalists and who proclaim self and world to be eternal” (sassatavādā sassataṃ attañca lokañca paññapenti); subsequently the theorist is introduced as stating his view in similar terms: “Eternal are self and world… they exist as eternally the same” (sassato attā ca loko ca… atthi idheva sassatisamaṃ). The last term appears likewise in our text; see Note 21. From this we may safely conclude that it is the identity, or unity, of the Self (or soul; mahātman, paramātman) with the universe (or the Universal Spirit, Brahman) which is conveyed by our text.
In the Commentary specific to our text, this eternalistic view is rendered and classified in the terminology of the Dhamma. The Commentary says:
“This statement (‘The universe is the Self’) refers to the (wrong) view ‘He considers corporeality, etc., as the self (rūpaṃ attato samanupassatī’ ti ādinā nayena).’”
The canonical quotation (e.g., in MN 44), included here in the Commentary, has two implications which are of importance for understanding the reason why it was cited in this context:
(1) As very often in the commentaries (e.g., to Satipaṭṭhāna Sutta), the term “world” (loko) is explained as truly referring to the five aggregates (khandhā, i.e., corporeality, feeling, etc.), singly or in toto.
(2) This quotation is the formula for the first of the twenty types of personality-belief (sakkāya-diṭṭhi; e.g., in MN 44). In the first five of these twenty, the self is said to be identical with each of the five aggregates (as in the earlier part of §15 of our text). Hence the application of this quote to our textual passage signifies that the theorist conceives the “world” (i.e., corporeality, feeling, etc.) as identical with the self.
The double “So (loko) so (attā)” in our text, should therefore, be taken as standing for “yo (loko) so (attā),” lit.: what is the world that is the self. In the Comy to MN 44 we find a similar phrase: “Someone considers corporeality as self: what is corporeality that is ‘I’; what is ‘I’ that is corporeality. Thus he considers corporeality and self as non-dual’ (… yaṃ rūpaṃ so ahaṃ, yo ahaṃ taṃ rūpan’ ti rūpañca advayaṃ samanupassati).” According to this interpretation the phrase has been translated here by “This universe is the Self.”
Mostly, the first five types of personality-belief are explained as referring to the wrong view of annihilationism (uccheda-diṭṭhi). [See, e.g., Paṭisambhidāmagga, Diṭṭhikathā, Ucchedadiṭṭhi-niddesa; further Comy to MN 44.]
But their being quoted in our context, shows that they may also apply to eternalism (sassata-diṭṭhi). We have come to this conclusion since it is improbable that, in our textual passage two mutually exclusive views should have been combined in a single statement formulating the sixth “ground for false views”; that is, in the first part of that statement, annihilationism, and in the second, eternalism. [Back]
23. “That I shall be after death…” (so pecca bhavissāmi). Comy explains by “so ahaṃ,” a Pali idiom, meaning literally “this I.” Pecca: lit. “having gone,” i.e., to the other world. [Back]
24. “Eternally the same” (sassati-samaṃ): an Upanishadic term; see Bṛhadāraṇyaka Upaniṣad 5.10: sāsvatīḥ samāḥ.
This entire statement of the sixth ‘ground for views’ may well have been the original creed of an eternalistic doctrine. The phrasing appears rather vague in the first part, and in general it is rather loosely worded (so for so aham). To contemporaries, however, the meaning may have been quite clear since it was perhaps the stock formula for teachings that were well known. Hence, in this translation, we have left the first part of the statement in its rather cryptic and ambiguous original form, while giving the interpretations in the notes only. [Back]
25. He identifies himself entirely (Sub-Comy: attānaṃ viya gaṇhāti) with that eternalist misconception (gāha), induced by craving (for self-perpetuation), by false views (tenaciously maintained) and by conceit (deeply ingrained ego-centricity). Here one view serves as subject-matter for another view (Comy, Sub-Comy). [Back]