Indeed it was klaf, the same as a sefer Torah.
The Rambam writes in Hilchot Sotah 3:8
Afterwards, a scroll of parchment from a kosher animal, like the parchment used for a Torah scroll is brought.
R Eliyahu Touger explains the Jerusalem Talmud (Sotah 2:4) states that the parchment must be made from the hide of a kosher animal, lest the woman refuse to drink and the passage be required to be entombed. It would not be fitting for God's name to remain on parchment from a non-kosher animal.
See also Hilchot Sotah 4:8 ("not to write on paper" because it is writen "ba'sefer") and Hilchot Tefilin 1:10 ("hides of [all] kosher animals, wild beasts, and fowl").