By what route did the letters of the traditional Mongolian alphabet reach it from the Aramaic abjad?

Why it is open. The descent is agreed in outline and unavailable in detail here. Mongolian took its alphabet from Old Uyghur, Old Uyghur took its own from Sogdian, and Sogdian belongs to the family descended from the Aramaic abjad. Whether the Sogdian alphabet was taken from the Syriac hand or directly from Imperial Aramaic chancery writing is argued about, and neither Old Uyghur nor Sogdian is held in this atlas as an alphabet, so no per-letter descent can be asserted from a Mongolian letter to any letter the atlas does hold. The letters here therefore carry intra-alphabet relations only.

What would resolve it. Seeding Old Uyghur and Sogdian as alphabets in their own right, with their letters, so that the chain can be recorded one step at a time rather than jumped in a single relation. Alongside that, a letter by letter comparison of dated Sogdian, Old Uyghur and early Mongolian documents against Imperial Aramaic and Syriac letterforms, taken from published palaeographic work rather than inferred.

Field. Palaeography and Central Asian philology