They're just URLs in the posts that used to either show an embedded external image or URL.
So here's an example, before the upgrade, the URL that's would have shown the file from the external website. Because it's http and not https, the converter seems to have skipped embedding it in 4.7.1 when rebuilding the content.
I've seen the same happen with youtube URLs where non-https youtube URLs just show as links after converting but https youtube urls are correctly embedded.
EDIT: I did also check as some of the ones that didn't convert are so old the file/video that was embedded no longer exists (so obviously there's nothing to embed), but the one in this example does still exist at that URL, and definitely would have been displaying as an image instead of a textual URL pre-upgrade.