Humm. Rob and Rosie should share this one as she was soooooo close at first guess. It is not technically a door lock, but can be used to lock a door with a hasp. " old locking mechanism" sort of covers a wide range of items.
It's the slider door on an old padlock as shown here.
"Rosie's first guess holds the key -- which is truly unique on this old item. "
DING DING DING .. I'll rule for Rosie... so she gets the next go. Okay Rob?