Set in Roman Magical Papyrus

Greco-Roman Period, Roman, 4th century (CE 300 / 350)
27 x 52 cm (10 5 / 8 x 20 1 / 2 in.)
Leiden Museum, AMS 75
Set detail: (translated from Dutch by Google)
"Figure with human body and Seth-animal head. His arms are stretched to the left and right and keeping big man standards. On his chest is "Seth" in Coptic letters, he wears a short skirt."
"Figuur met menselijke lijf en Seth-dier hoofd. Zijn armen zijn naar links en rechts uitgestrekt en houden mansgrote standaarden. Op zijn borst staat "Seth" in Koptische letters, hij draagt een korte schort."
Larger view underneath - Photos © Monique van der Veen

Museum photo which places the Set figure in context

_The Leyden papyrus: an Egyptian Magical Book_, by Francis Llewellyn Griffith, F. Griffith, Herbert Thompson, page 13:
"The use of hieratic might be thought to indicate some antiquity where it occurs. But the writing is a strange jumble; the hieratic is inextricably through sparingly mixed with the demotic, a single word being often written partly in hieratic, partly in demotic, a single word being often written partly in hieratic, partly in demotic. Where hieratic signs occur the language is not generally more archaic than when the demotic is pure."

I found a drawing of this donkey-headed Set in The Greek Magical Papyri in Translation edited by Hans Dieter Betz, along with a translation of the spell:

It is in PGM XII. 449-52, which is a "spell of separation", in which the goal is to "Separate NN, born of NN, from NN, born of NN!" (page 170)

An interesting passage from the Leiden Papyrus:

"Soul of souls, Bull of the night, bull (?) of bulls,
son of Nut, open to me, I am the Opener of earth, that came forth from Geb, Hail!"
(One of Set's titles is 'Son of Nut', and he is often called 'Bull', as in 'Bull of Ombos')