Hi Doug, other than the "4096" stamp, the magazine appears to be a standard fxo that has had the plastic bottom damaged. While the post war magazines were made with the same equipment as the fxo, they didn't contain the fxo or E/37 markings.
My guess would be that someone wanted to identify the magazine to a pistol and stamped the serial number on the magazine. I am also reasonably sure that it didn't leave the factory that way.