Interestingly this worked for about half of the ebooks with this problem. When I changed the extension to .pdb, Calibre could import them but it used Stanza to open the files, which is fine.
For the other half Calibre used Stanza to try to open them, but Stanza gave an error message as well ("Cannot open locked file"). I took one of them and tried the new Barnes & Noble reader app, and it opened it without problems! Go figure. In any case, this is what the header looked like at the top of the ebook:
"<HTML><HEAD><meta name="GENERATOR" content="BeMobi MobiBook Publisher 1.0"><TITLE>The Wealth of Nations</TITLE></HEAD><BODY>", [The Wealth of Nations]
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