M. de Dunblan is the way the first known Bishop of Dunblane is written in a copy of a papal bull of Pope Adrian IV preserved in England; the bull dates to 1155.[1]
The papal bull was addressed to the bishops of Scotland ordering them to submit to the metropolitan authority of the Archbishop of York; the copyist made two other mistakes in the initials of bishops, so it is not totally reliable.[2]
Cockburn speculated that M. might stand for Máel Ísu;[3] it is very unlikely that M. was a mistake for La., standing for Laurence the successor of M. at Dunblane.[4]