Hello.
The distinction follows Brazilian cataloging criteria (where the main references are Bentes and Amato).
The SCWPM distinguished P#176c and P#176d only by the signatures; however, there was an intercalation of series periods between these two variants (as can be seen in the annual lines).
For a common non-Brazilian collector, these variations are not so important, with one unit of P#176c and one of P#176d in their collections being enough. But for those more advanced in Brazilian banknotes, this separation is widely cataloged.
Regards,
Vladimir.