it is a module containing pre-defined calculation routines especially for Resistor/capacitor networks/filters. It comes with a book and keyboard overlays where the redifined keys are shown.
I have one of those for my HP-41CX, and it announces itself as the Circuit Analysis 1A pac. Looks to have the same content as the Circuit Analysis pac available here at HP41.ORG (interesting that it is considered 00041-15006 instead of the number on the module itself).