The Medieval Latin abbreviation for York (Eboracum), used instead of his surname in the signature of an Archbishop of York. For example, the present incumbent, Dr David Hope, would sign David Ebor.
(Likewise the Archbishop of Canterbury signs Cantuar instead of his surname, and several bishops also have distinctive Latin signatures.)
The names Eboracum and York and the Danish name Jorvik (York used to be an independent Viking kingdom during the Anglo-Saxon period) are all related in etymology, I believe.