Define "ad indicem" in bibliography

According to this page, which seems reliable, it means vedere l'indice ("see the index"), which makes sense if you consider the most obvious translation of the Latin ("at (the) index"). The expression appears to be common in Italy.

The author of your first reference seems to use it when he doesn't provide any specific page numbers to refer to. His article is titled Paul IV, so what he wants you to do is go to the index of the work in the reference, find "Paul IV", and consult the page number(s) you find there. It is a bit of a lazy way of referring: he can't be bothered to look up the page numbers where the reader can check his reference, but he doesn't think that is necessary, because they are easy to find by that method. It would seem that ad indicem only works if the referring text is clearly about a single topic that can be easily and unambiguously looked up in an index, like "Paul IV".

Note: the translation "to the index" is possible in theory, but it would not be an obvious translation.

I can't find a definition for this, but according to Google Translate, "ad indicem" means "the index".

Looking at the context, my guess would be that it means "see the index for more details".

EDIT - as @Chappo points out in the comments, it may mean "to the index" rather than simply "the index", in which case it could perhaps be viewed as an instruction to the reader, ie to go to the index to find more information.