Donald W. Parry notes that the Great Isaiah Scroll has "these" (masculine plural) in the text of Isaiah 51:19; a scribe believed "two" in verse 19 referred to "sons" from verse 18.
Donald W. Parry, Exploring the Isaiah Scrolls and Their Textual Variants (Supplements to the Textual History of the Bible 3; Leiden: Brill, 2020), 363
הֵנָּה
MT’s independent f. pl. pronoun הֵנָּה refers to שְׁתַּיִם; 1QIsaa’s המה is in error, or perhaps a copyist thought המה refers back to בנים. (v. 18)