It looks to me like a replaceable core for a commercial lock. This type of core has a core removal key. When the core is placed into the lock the removal key is then turned and the small metal bar on the side of the core swings out to hold the core in place. Once this is done the removal key is taken out and the subkey can then be used to open the lock. These have been used for years in buildings where rekeying locks happen on a daily basis. Schools, hospitals, etc.
It's a file commercial lock. Best makes several like this. If you can't find it on the web pm me & I'll give you more info. Not close to my key catalogs tonight.