What is the structure of the “Featured Image” field in the DB?

Featured images got messed up after an import with the “media tools 1.1” plugin. The plugin wrongly designated (in the DB) the incorrect filename for the images: Instead of pointing to the correct file, it appended a “1” to its name, causing, for example, a 404 for “Protest-11-620×264.jpg”, when it should point to “Protest-1-620×264.jpg”

I need to do a search & replace in the DB on *_wp_attachment_metadata* field and remove the extra “1” from the filenames. Naturally I will use a regex, but how do I process the contents of the field? Which one of the sections of the data has which featured image to use? In other words, what do I need look for to know the filename is the right one to search&replace?