Brewer's: M. B. Waistcoat
A clerical cassock waistcoat was so called (about 1830) when first introduced by the High Church party. M. B. means “mark of the beast.”
“He smiled at the folly which stigmatised an M.B. `waistcoat.'” —Mrs. Oliphant: Phoebe Juno, ii. 3.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894