Ṣedeq and El Elyon ("God most high") may have been two epithets of the same Jebusite god, identified as an astral deity, perhaps eponymous of Salem itself: Salim or Shalem (שלם) is attested as a god, presumably identified with the evening star, in Ugaritic mythology; URUŠalim in this case would be the city of Salim, the Jebusite astral deity.
The name of the group (and of the siblings) is an allusion to Melchizedek, a figure from Christian and Jewish tradition.
Others still maintain that Melchizedek is actually Archangel Michael: Michael is designated in the apocryphal Book of Enoch and the canonical Book of Daniel as "the prince of Israel".
In the novel The Alchemist, by Paulo Coelho, the king of Salem gives the main character Santiago two stones that the king calls Urim and Thummim.