Sir Richard C. Jebb, Commentary on Sophocles: Oedipus at Colonus

αὐτοπέτρου βήματος, a "step," i.e. ledge, of natural rock, not shaped by man (as was the ordinary “βῆμα” or raised place for speakers, etc.), distinct, of course, from the “ἄξεστος πέτρος” of 19, which was within the grove. So “αὐτόξυλος” (of rough wood, Ph. 35), “αὐτοπόρφυρος” (of natural purple), “αὐτόποκος” (of simple wool), “αὐτόπυ_ρος” (of unbolted wheaten flour), “αὐτόκομος” (with natural hair, Aristoph. Ran. 822), “αὐτόροφοι πέτραι” (rocks forming a natural roof, Oppian Halieut. 1. 22). The ἀντιπέτρου of the MSS. could mean:—(1) "A ledge like rock"; cp. “ἀντίπαις” (Aesch. Eum. 38)= "weak as a child": and so the schol. in L, “ἰσοπέτρου, χαλκοῦ”,—i.e. "a ledge of material firm as rock," "of brass," meaning the “χαλκόπους ὀδός” understood literally: see, however, on 57. (2) "A ledge serving as a rock": cp. (“ὀνείδη”) “ἀντίκεντρα” (Aesch. Eum. 136), “λίθος ἀντιθύρετρος” (Nonnus 11. 140), “ἀντίπυργος πέτρα” (Eur. Bacch. 1097). (3) "A seat of rock fronting thee": cp. “ἀντίπρῳρος”, with “πρῴρα” facing one. This does not fit the data. (4) Bellermann: "a (stone) seat over against a rock," i.e. "behind which the stone wall rises" (?).—Campb. renders first by "rocky," then by "rock-like," and refers it to “"some peculiarity in the basement of the low seats."”