In a previous paper Bingenheimer attributed SA-2 to comparatively northerly Central Asian Sarvāstivādins and SA to southerly Indian Sarvāstivādins, arguing they originated as more or less the same document with SA-2 being translated in a northerly Prākrit. I’ll look for the citation in a second. I wonder if he still favours that proposal.
EDIT: here, The literature labelled SA & SA-2 here - #3 by Coemgenu, just ignore my very amateur Chinese work! 
