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I think you mean what's usually written MN I 436 (roman one, no dot)... These are volume and page number in the Pali Text Society official printed Pali edition of the Canon.
You can see them e.g. on suttacentral.net next to the sutta number.
Looks like MN I 436 is somewhere between the beginnings of MN 64 and MN 65, most likely it refers to a quote on the last page of MN 64, as found in the official PTS Pali edition.
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Also there's a cross-reference between the two numbering systems, on this page:
Sutta and JΔtaka number and name, links and PTS page number
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Ven. Bodhi's book also provides PTS references inside square brackets. For the MN 64 example, his book will show:
"Whatever exists therein of material form, feeling, perception, formations, and consciousness, he sees those states as impermanent, as suffering, as a disease, as a tumour, as a barb, as a calamity, as an affliction, as alien, as disintegrating, as void, as not self. He turns his mind away from those states [436] and directs it towards the deathless element thus..."
Else as Andrei mentioned, simply go to suttacentral and click MN 64, then click the cogwheel icon on the upper right hand side (it'll say "Text settings" when moused over), then click the "View textual information" slider and it'll shows the infos. on the right hand side of the paragraphs.