Does anybody recognize this kind of prism?
My guess is that it's not a prism (or at least doesn't function as one optically).  It looks like it is a right triangle in one cross section.  The diagonal face would be silvered to act as a star diagonal. If aligned correctly, the black plate it's glued to that attaches to the support arm would be perpendicular to the tube axis.

It looks like it's rough beveled on other sides, maybe to get an ellipsoidal cross section?