An OPF preamble does seem to optional. This is from the MobiPocket version of
Ring of Fire from the Baen Free Library.
Code:
<HTML><HEAD><metadata>
<dc-metadata xmlns:dc="http://purl.org/metadata/dublin_core" xmlns:oebpackage="http://openebook.org/namespaces/oeb-package/1.0/">
<dc:Title>Ring of Fire</dc:Title>
<dc:Type>Novel</dc:Type>
<dc:Identifier id="ISBN-074347175X" scheme="ISBN-Hardcover">0-7434-7175-X</dc:Identifier>
<dc:Identifier id="ISBN13-9780743471756" scheme="ISBN13-Hardcover">978-0-7434-7175-6</dc:Identifier>
<dc:Identifier id="ISBN-1416509089" scheme="ISBN-Paperback">1-4165-0908-9</dc:Identifier>
<dc:Identifier id="ISBN13-9781416509080" scheme="ISBN13-Paperback">978-1-4165-0908-0</dc:Identifier>
<dc:Identifier id="DOI-074347175X" scheme="DOI">10.1125/Baen.074347175X</dc:Identifier>
<dc:Publisher>Baen Books</dc:Publisher>
<dc:Creator role="aut" file-as="Flint, Eric">Eric Flint</dc:Creator>
<dc:Contributor role="art" file-as="Blair, Dru">Dru Blair</dc:Contributor>
<dc:Subject>Science Fiction</dc:Subject>
<dc:Rights>2004 by Eric Flint</dc:Rights>
<dc:Date>2004-01-01</dc:Date>
<dc:Language>US English (en-us)</dc:Language>
</dc-metadata>
</metadata>
<GUIDE>
<REFERENCE TYPE="toc" TITLE="Table of Contents" HREF="074347175X_top.htm" filepos="0001692887">
<REFERENCE TYPE="cover" TITLE="Cover" HREF="074347175X__i_.htm" filepos="0000001553">
<REFERENCE TYPE="copyright-page" TITLE="Copyright" HREF="074347175X__p_.htm" filepos="0000001785">
<REFERENCE TYPE="firstpage" TITLE="First Page" HREF="074347175X__p_.htm#Chap_0" filepos="0000004946">
</GUIDE>
<METADATA HREF="xyz_metadata.htm" filepos="0001694500"><hr></HEAD><BODY>
<h1 align="center"><img src="BMP" recindex="00001"><br />
Ring of Fire<br />
by<br />Eric Flint</H1>
<p align="center"><A HREF="074347175X_top.htm" filepos="0001692887">Table of Contents</A></P>
If the e-book has anything more than the first page under the Reader's contents icon, then I think it has to have a non-empty <GUIDE> section.
Another way to generate "typical" MOBI books would be to run mobigen.exe on an exploded LIT file, and compare the result to using html2mobi. In the case of Baen books, you can use the LIT version and compare the result to their MOBI version.