These are HTTP ETags—via Wikipedia:
An ETag (entity tag) is an HTTP response header returned by an HTTP/1.1 compliant web server to determine if the URL cached is identical to the requested URL on the server. The header is useful for intermediary devices that perform caching, as well as client web browsers that cache results. One method of generating the ETag is based on the last modified time of the file and the size of the file.
I’ve wondered what these were before but didn’t know exactly until a friend sent me the above link.
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