Lavrentivs wrote:The apparatus of Aristotelis Physica (ed. Ross, 1950) has the letter M. e.g., ‘om. MS’ I take to mean ‘omiserunt M et Simplicius’; but there is no mention of any M in the praefatione . . . ?
You seem to be asking two separate questions. Is it M or MS you can't fathom out? In the absence of further context, MS would, as Qimmik points out, ordinarily stand for "manuscript". The abbreviation M is listed in the Sigla facing the first page of text in the OCT; we are told it stands for "Aristotelis Metaphysica
". On what page of the OCT do we encounter "om. MS"?
OK, a bit of an update. I've found an instance of "om. MS" in the apparatus in reference to the word κινητῷ at 200b, line 32. The word κινητῷ is in square brackets in the text, which means Ross thinks the word is out of place and should probably be deleted.
"om. MS" after κινητῷ in the apparatus means that both a corresponding passage in the codices of the Metaphysics and a citation of this passage in Simplicius' commentary also omit the word.
See the first paragraph of the Praefatio for Ross's explanation of the value of the Metaphysics codices and Simplicius' commentaries in establishing an accurate text.