Here is Tischendorf writing about the three crosses note, in the translation of his 1850 book on the Septuagint:
Vetus Testamentum graece juxta LXX interpretes (1850)
Prolegomena p. LI
Charles Short (1821-1886)
Bibliotheca Sacra and Theological Review, Volume 10 (1853)
PROLEGOMENA TO TI8CHENDORF'S NEW EDITION OF TIIE SEPTUAG1NT.
Translated from the Latin by Charles Short, M. A., Roxbury, Mass.
[Concluded from Vol. IX. p. 608.]
1852 first section is at:
But how it happened that the same correctors did not even touch the first four leaves of the books of Chronicles, though the note testifies that these books were contained in the copy of Pamphilus, may be learned from what is found written by the third hand at the bottom of the fourth leaf, with the sign of a triple cross affixed. This is as follows:
μεχρι του ϲημειου τω τριων ϲταυρων εϲτι το τελοϲ των επτα φυλλων τω περιϲϲων κ(αι) μη οντω του εϲ δρα
Greek transcription from the Codex Sinaiticus Project.
On the manuscript it goes like this:
μεχρι του ϲημειου τω
τριων ϲταυρων εϲτι
το τελοϲ των επτα
It is thus testified that these four leaves, along with three others long since destroyed, were not so much inserted in the codex in an improper place, as introduced by mistake on the part of the copyist and perhaps twice written on. It was for this reason, I think, they were passed over by the reviser.
Here is the note on the manuscript, bottom of the 4th column (it originally started higher on the column but was moved down.)
1 Chronicles (duplicate), 18:15 - 19:17 / 2 Esdras, 9:9 - 9:11 library: LUL folio: iv_v scribe: A
https://codexsinaiticus.org/en/manuscri ... omSlider=0
Tischendorf's explanation is simple and sensible, this was a scriptorium note acknowledging the scribal blunder.
This post, with some PICS included, is at:
Pure Bible Forum
three crosses note explained by Tischendorf in his 1850 LXX book, translated by Charles Short
https://www.purebibleforum.com/index.ph ... hort.2844/