It's a box-end wrench, it's a socket--no, it's a Boxocket!
I found one of these recently in Ottawa. According to Alloy Artifacts, this model (X16) was made in 1927. (I think it's cool that this tool was made the same year that Lindbergh crossed the Atlantic!) The "Boxocket" series itself was introduced in 1926 or 1927 and usually sold as sets. Apparently, these were among the first double-box wrenches available from any manufacturer. The Boxocket name was apparently continued into the 1950's. Anyway, mine has been cleaned up and added to my other wrenches so it can be put back to work.