I have looked up some printed sources and I agree with Michael Everson and Frédéric Grosshans that the
beast in question is a variant of the greek letter tau (capital or lowercase).
Here are the relevant sources I consulted:
Carl Faulmann: Das Buch der Schrift. Enthaltend die Schriftzeichen und Alphabete aller Zeiten und aller Völker des Erdkreises. Verlag der kaiserlich königlichen Staatsdruckerei. Wien 1878, 2. verm. und verb. Aufl. 1880 p.171
Hans Jensen: Die Schrift in Vergangenheit und Gegenwart, 3. Auflage p.459
Here is a quote from Hans Jensen:
Noch in modernen Drucken finden wir die Formen ϐθϖ3ϲ7, wo andere βϑπζςτ haben.
Note: i had to fake the zeta symbol with a digit 3 and the tau symbol with a digit 7 here. In German typesetting tradition the theta symbol ϑ is the preferred form, not the straight theta θ.
My Opinion: The Greek Zeta Symbol and the Greek Tau Symbol are on the same footing as the "lunate sigma" alreay encoded in Unicode. They should be added in both lowercase and capital form.
--Jörg Knappen