Originally Posted by mbovenka
True, PRC files *can* be many things. But if they're in a Calibre library, I think it's a good bet they are books, and thus in effect Mobi files
I imported my ebooks a few months ago (so not with 'a really old version of Calibre'), and there are lots of 'PRC' format books in my library. Calibre does not rename them, and treats them as a different format from Mobi. I'm not really sure why that is; I know modern Mobi files have a more extensive header than old PRC files do, but AFAIK they are basically the same thing (i.e. the PRC header is a subset of the Mobi one). I'd think the Pagecount plugin should be able to handle them simply by treating them as Mobi files, but perhaps it's trickier than I know.
Perhaps Kiwidude 'll chime in on that; he should know.
'Really Old' does appear to be a slight falsehood - the last book in my library that is a real mobi with a PRC extension is from December 2010, so it must have been a release that month that changed the logic. I've been regularly adding prc files since that time (just did some today) and they all have a .mobi extension after Calibre processes them. Still, seven months is a while.
I still don't think it's safe to assume that prc is mobi - Calibre supports a number of ebook formats that were popular on PalmOS, mobi just happens to be the most popular one.