A number of markings and corrections were called "medieval" and assigned to correctors D and E.

"Later and unimportant correction hands are D and E" - Dirk Jongkind​

"The medieval D and E correctors are of slight importance." - Skeat & Milne
Corrector D is dated by Tischendorf to the 8th or 9th century, E to the 12th. However, these are Tischendorf dates, so they are not really based on anything substantive. We will look at a few of the spots here.

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Scrivener never saw the manuscript, so he gives us the info from the Tischendorf publication.

A full Collation of the Codex Sinaiticus with the received text of the New Textament (1864)
https://books.google.com/books?id=v-JUmBD5zIcC&pg=PP33

"Whose style bespeaks the 8th or 9th century ... D" - p. xxv

E appears but three times in the New Testament, and seems fully as late as the twelfth century. - p. xxv

"corrections .. those indicated by D and E look very black, as in Tischendorf’s Facsimiles" - p. xxxi
Corrector E places the word "God" in 1 Timothy 3:16. "God was manifest in the flesh." ...