Seth. . . originally connected with the Hyades, the V-shaped, head-like part of our constellation, TAURUS. As the brother of Osiris, his position in the sky was adjacent to ORION. . .an important court decision gave the office of Osiris to Horus, and Seth was banished to a position bearing the 'southern' constellation of ORION. . ." - Jane B. Sellers, The Death of Gods in Ancient Eqypt

Bauval and Gilbert 1994:138

These rites were extremely ancient and are described in the Pyramid Texts:

'O King, I have come in search of you, for I am Horus; I have struck your mouth for you, for I am your beloved son; I have split open your mouth for you. . .with the adze of Upuaut. . .with the adze of iron. . .'[PT 11-13] "...your children's children together have raised you up,[namely] Hapy, Imsety, Duamutef and Kebhsenuf, [whose] names you have [wholly] made. [Your face is washed], your tears are wiped away, your mouth is split open with their iron fingers...'[PT1983-4]
There are three important aspects of this rather bizarre ceremony which demanded our undivided attention. The first was that the adze instrument and also the fingers of the four sons of Horus are said to be made of bja (meteoritic iron). 40 This was picked up by G.A. Wainwright in 1931, and discussed in detail in a landmark article entitled 'Iron In Egypt'. 41 Wainwright rightly argued that it was because of the 'heavenly' qualities of bja that the ceremony was believed to evoke the magic for the escape of the soul to the stars.42 This is now a well-accepted notion in Egyptology, and was recently repeated by Dr Bernd Scheel, an expert in ancient Egyptian metal-working and tools, who wrote:
Iron was [a] metal of mythical character. According to legend, the skeleton [bones] of Seth...was of iron. Iron was called the 'metal of heaven' because for a long time the Egyptians knew only meteoric iron, which has a high nickel content. Because of its supposedly divine origin, meteoric iron was used in particular for the production of protective amulets and magic model tools which were needed for the ritual called the 'opening of the mouth', a ceremony which was necessary to prepare the mummy of the deceased for loife after death.43
Bauval and Gilbert 1994:205-206