I wouldn’t totally rule out the possibility of a connection with the railways/railroads.
Its too basic, bland and featureless to be anything secure or for decoration or display, but its overall size, large bow and small blank bit are characteristics sometimes seen on some basic railroad keys.
Between carriages, wagons, gangway connectors, signals, points etc just about every shape and size configuration was used for the simpler budget action locks.
Keys for lower risk but heavy use were more toolish with Tommy bar handles and basic tapered square shafts, while at the other end of the scale there were some like yours with large bows and small bits still operating a simple sprung action click-to-lock and unlock, but the additional projecting keybit preventing use of other simple shaft keys.
Thats not to say it is though- it could even be as simple as an old discarded keyblank that had the bit cut down sufficiently long ago to age and patinate evenly with the rest of the key!
