Before the invention of chapter and verse ancient writers alluded to scriptures they had in mind by citing something in or taught by it. 666 is found pointing TWICE to one particular name, with a little “counting” and by the gentitive in the text:
the sons of Adonikam, 666 (Ezr. 2:13 NAS)
the sons of Adonikam, 667 (Neh. 7:18 NAS)
John wasn’t like modern textual critics, he would believe every word of inspired Scripture is correct. Therefore, John would conclude this anomaly was divinely intended, the difference in number for the very purpose of his riddle, to point to the name of the Antichrist using two lines of evidence. This would establish the matter as it is written from the mouth of two witnesses a thing is established:
Adonikam had 666 or 667 sons. Therefore, when we “count” (5585 ψηφίζω psephizo) “as with pebbles” we have 667-1=666, arriving at the same name Adonikam. 666 is also “the number of a man” because firstborn Adonikam had “666 sons”, they are “OF” him.