MB in square indent is mentioned in J. Gavin Scott's British countermarks on copper and bronze coins (Spink 1975), sourced to Spink's Numismatic Circular Nov./Dec. 1920. It is applied to an otherwise unreadable halfpenny. The book does not cover silver.
Scott also mentions MB stamped incuse in a 1750 halfpenny.
Neither issuer is known.
Peter