An unidentified Italian publisher's device: the knight on oxback

IN November 1891 the British Museum bought from Leo S. Olschki, the bookseller who was at that time established in Venice but later moved to Florence, a small liturgical work in 16mo format without imprint or date, and with the title printed in red as follows (abbreviations resolved): Diurnum Romanum: Cum capitulorum quotationibus: ac foliorum numero in margine ad omnia que in hoc diurno continentur faciliter inueniendum. At the top of the title-page, in black, is a roundel containing the Virgin and Child with five stars; and at the foot of the title-page, also in black, is a device, measuring 30 mm. high by 34 mm. wide, and showing a knight holding aloft in his right hand a sword, and riding on an ox. The device contains the letters Z.M. at the top and B.B. at the bottom. This device has never been identified by the British Museum catalogue, which still has a footnote reading: 'On the title page is a printer's device with the initials Z.M.'