M. de Dunblan

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.
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.
Cockburn speculated that "M." might stand for Máel Ísu; it is very unlikely that "M." was a mistake for "La.", standing for Laurence the successor of "M." at Dunblane.