Redating an ex years later
There appears to be no surviving contemporary evidence concerning the grant of the earldom.
However, Balfour Paul refers to a charter "noted in an old Inventory of Oliphant writs, made about 1594, and preserved in the General Register House" which states that King Alexander II granted "the erledom of South Kaythnes" to "Magnus sone to Gylcryst sum tyme erle of Angus".
The Complete Peerage says that Gillbride seems to have married a daughter of Gospatrick Earl of Dunbar but does not specify the primary source on which this is based --- of Caithness, daughter of ERIK Slagbrellir & his wife Ingigerd Kalisdatter.
According to the Complete Peerage, Gillbride Earl of Angus married as his second wife "the heiress of the earls of Caithness", in another passage stating that she was "sister of Harald Ungi Earl of Caithness".
The inventory note does not date the charter in question.
However, the grant, if the report is accurate, must have taken place after 1231, the date of the death of Earl John, last of the previous lines of earls of Orkney and Caithness.
It does not cite the source on which this statement is based and, as discussed further above under the possible second wife of Earl Gilbride, the hypothesis appears to be entirely speculative.
The separate primary source references to an individual named Magnus indicate that this suggestion is probably incorrect.
From a chronological point of view, Magnuss estimated birth date range as shown above suggests that he was either the son of Gillbride Earl of Angus by a second marriage or that he was the son of Earl Gilchrist.
Gillbride was the first individual to be styled (in 1135) Earl of Angus and members of his family succeeded him in the title until the mid-13th century. The Pictish Chronicle names Conchar Mormaer of Angus as father of Fynebole Lady of Fettercairn, who murdered Kenneth II King of Scotland in 995 in revenge for the slaughter of her only son oat Dunsinane. The Chronicle of the Scots and Picts dated 1177 records that "Kynnath mac Malcolm" reigned for 24 years and 2 months, was killed "a suis hominibus in Fetherkern" through the treachery of "Finuele filie filie Cunthar comitis de Anguss" whose only son had been killed by the king.
The Pictish Chronicle names Conchar Mormaer of Angus and father of Fynebole Lady of Fettercairn who murdered Kenneth II King of Scotland in 995 in revenge for the slaughter of her only son oat Dunsinane . "Alexander nepos regis Alexandri, Beth comes, Gospatricius Dolfini, Mallus comes, Madach comes, Rothri comes, Gartnach comes, Dufagan comes, Willelmus frater regine, Edwardus constabularius, Gospatricius filius Walthef, Ufieth Alfricus pincerna" witnessed the charter dated to [1114/15] under which "Alexanderrex Scottorum filius regis Malcolmi et regine Margerete et Sibilla regina Scottorum filia Henrici regis Anglie" reformed Scone Abbey, son of --- (-). David I King of Scotland granted protection to the clerics of Deer by undated charter witnessed by "Donchado comite de Fib et Malmori d Athotla et Ggillebrite comite d Engus et Ghgillcomded Mac Aed"] [--- of Dunbar, daughter of GOSPATRICK Earl & his wife ---.
Six of these local rulers are for the first time called "comes" in the foundation charter of the monastery of Scone dated [1114/15].