I think the pagebreak constructs that look like
Code:
<div class="mbppagebreak" id="calibre_pb_16"></div>
are what you get when you convert an old-style MOBI using calibre. I used the 'Kindle Unpack' plugin to unpack a few of my old Amazon original MOBIs at random and the source code looked like this in all of them
ETA: As the unpacked markup code is all in a single file I'm assuming calibre uses them to decide where to split into multiple files.