Adult Rebecca is left handed and teen Rebecca is right handed.
The teenage Rebecca Vanderpool (Tiger Darrow) is right-handed as seen writing in her diary throughout the flashbacks, but the adult Rebecca Vanderpool (Anne Ramsay) is left-handed, seen writing in the same diary towards the end.
When adult Rebecca Vanderpool is working on her outline on the white board you see "visit Jack The Ripper" written with the date 1799 above it. The Jack The Ripper murders were in 1888.