Better late than never in my response - I hope.
This type of plate was fitted as an access to the lock on the rear door pan of a range of fire-resisting products which Chatwood produced just prior to the last war to counter the successful imports from the US of the “Library Bureau” range of fire resisting equipment.
The new products included book room doors, document cabinets, filing cabinets, fall-front cabinets (Kardex), planfiles etc. These differed in design from anything they had previously produced. They were constructed of light bent steel outer casings and door skins using ‘pressed steel corner bindings’ to give greater resistance to impact in building collapse. The inner casing was of a heavier construction.
The fire-resisting material was in the form of aerated cement with wire mesh reinforcement.
The doors and frames were double rebated as a fire seal. They were usually finished in black.
The principal was covered by their 1933 Patent 429628 which also included an improved boltwork system (espagnolette type) to help prevent the doors from bursting open on impact.
Safeman.
