I have looked at both the collected works of Gauss and at the English version of the Theoria Motus, in order to see what a later editor made of this symbol.
In the Werke the symbol ’7’ continues to be used : C F Gauss, Werke, Vol. 7, ed. E J Schering, Gotha, 1871; § 77, M = N + n’7’ ̶ Π. In the translation the ‘7’ is replaced by the lower case tau. Theory of the motion of the heavenly bodies moving about the sun in conic sections: a translation of Gauss's "Theoria motus." With an appendix. By Charles Henry Davis, Boston : Little, Brown and company, 1857; § 77, M = N + nτ ̶ Π. So this seems to settle the matter of the identity, and just leaves one to puzzle over the German use of this sign for tau. Raymond
