The Super-Id is the mental representative of a primitive phase (pregenital, prehistoric, pre-oedipal), during which morality had not yet been separated from the pleasure principle and set under the sway of the reality principle.  This phase ends with the introjection of external authority, forming the superego.  This occurs simultaneously with the alienation of the ego’s authenticity, establishing the ego-ideal.  The superego thereupon begins continually to punish the ego for not living up to the (alienated) ego-ideal.  The id is allegedly the repository of all primal aggressive and sexual impulses.  At present the ego converts much of the id’s energy, devoting this to the very repression of the id.  Since the id is not entirely separate from the ego, that also represents a self-punishment, in the service of the super-ego, for the ego’s own failure to attain the standards of the ego-ideal.
Socialization occurs under the sway of the reality principle, and in opposition to the pleasure principle.  However, the pleasure principle under which the Super-Id enjoys, antedates even the phase of polymorphous perversity.  That principle may be construed as a “norm”; in that instance, there is no “structure” to speak of; and the Super-Id is that of which the functioning is recognized as, and called, undifferentiated infantile narcissism.The pleasure principle is, primordially, the eternal availability of the nipple such that it is not recognized as another’s.  The reality principle, primordially, is that this is not immediately always available upon demand and that there is consequently in fact (an)Other other than (one)Self.
The Super-Id is a “missing link” that is the ground for re-unifying the superego with the ego-ideal.  However, the Super-Id does not abstractly reunify the superego with the ego-ideal; rather, it brings the superego over to the side of the ego.  In this manner the falsely established alienation of ego from ego-ideal dissolves.  The internalized persecution of the ego halts, liberating the energy of the id in constructive directions.
The Super-Id is mentioned in passing in Herbert Marcuse, Eros and Civilization, with the original German reference from early this century.  Of possible further interest are Lacan; and René Girard, Violence and the Sacred.