Richard Gordon suggests the object on the Miles step is a bull’s hindquarter.
“In the light of the sacrificial scene on the altar of Flavius Aper (Poetovio), the interpretation as a bull’s hind-quarter rather than shoulder is to be preferred. The scene at Ostia is perfectly in keeping with other evidence suggest- ing that (junior) Mithraic grades fulfilled specific manual tasks within the cult, in the case of Miles, butchery of sacrificial animals.”
Gordon, R. 2013c. “The Miles-frame in the Mitreo di Felicissimo and the practicalities of sacrifice.” Religio: Revue Pro Religionistiku 21, no.1: 33–38.
The Mithraeum Felicissimus has a floor mosaic depicting the seven mithraic grades.