Ok, dwanthny was kind enough to send me an example epub - thx very much! What I hypothesised was correct. The older style jackets had no <meta> tags.
I'm looking at the jacket.xhtml file and wondering what might distinguish this as a Calibre jacket versus one that the original epub author coincidentally called jacket.xhtml. I can see why Kovid added the <meta> tag... lol
I welcome any suggestions... below is the jacket (dwanthny I have removed any identifying content in case it was a problem).
Code:
<?xml version='1.0' encoding='utf-8'?>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>Title of the book</title>
<meta content="http://www.w3.org/1999/xhtml; charset=utf-8" http-equiv="Content-Type"/><link href="stylesheet1.css" type="text/css" rel="stylesheet"/><style type="text/css">
@page { margin-bottom: 5.000000pt; margin-top: 5.000000pt; }</style></head>
<body class="calibre2">
<div class="calibrerescale">
<div class="calibre3">
<h1 class="calibrerescale1">Title of the book</h1>
<h2 class="calibrerescale2">Book Jacket</h2>
<div class="calibrerescale"><b class="calibre4">Series: </b>My series [4]</div>
<div class="calibrerescale">Rating: <img alt="star" src="star.png" class="calibre5"/><img alt="star" src="star.png" class="calibre5"/><img alt="star" src="star.png" class="calibre5"/><img alt="star" src="star.png" class="calibre5"/></div>
<div class="calibrerescale"><b class="calibre4">Tags: </b>Mystery & Crime</div>
</div>
<div class="calibrerescale">
SUMMARY: Some summary
</div>
</div>
</body>
</html>