Balfour Pauls Scots Peerage says that "Magnusis usually designed son of Gillebride Earl of Angus", adding that the "statement was first made by Sir James Dalrymple in his Collections, but he gives no proof".The Complete Peerage says that "it seemsquite probable that [Magnus] was the same person as Malcolm Earl of Angus, son of Duncan, son of Gilchrist, son of Gillbride[who] is named as Earl of Angus and Caithness in 1232 [see above]", although conceding that "the whole matter is, however, very obscure".The chapters on the Scottish earls and lords which are set out below are arranged in approximately chronological order of creation of the titles.The first group of seven earldoms, Angus, Atholl, Caithness, Fife, Mar, Moray and Strathearn, corresponds to the seven provinces into which Scotland north of the Firths of Forth and Clyde was divided in the 9th century, reputedly ruled by seven brothers.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.
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.
No primary source is cited in support of these statements and it is possible that this marriage, and the supposed parentage of the bride, are entirely speculative, in an attempt to explain the transmission of the half of the earldom of Caithness to Earl Magnus (see below).
Skene says that "the probability is that the half of Caithness which belonged to the Angus family was that half possessed by the earls of the line of Erlend, and was given by King Alexander with the title of Earl to Magnus, as the son of one of Earl Harald "Ungi"s sisters" and that "the Norwegian name of Magnus indicates that [Earl Magnus] had a Norwegian mother".
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.