Originally Posted by ldolse
PRC can actually contain many things, as it's originally a PalmOS extension/container format. Anyway Calibre looks inside the PRC container during import and if it's really a mobi file it changes the extension to mobi. If you have prc files in your library that are actually mobi it's because you imported these with a really old version of Calibre.
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.