Door locks are manipulated to open or close with brass keys. Keys work by duplicating the interior of a lock's millings and grooves. This produces the metal "hills and valleys" we see on keys. Most of them are stamped with the manufacturer's name and/or codes. Each code is unique to the particular key model and helps locksmiths identify the lock brand if unknown.
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Look on the head or bow of the key; this is the large, blunt end of the key. Lock makers often stamp their names on the bow or head of the key.
Take the key to a locksmith or hardware store. Both have reference books that identify keys and the lock brand.
Ask the locksmith or hardware store operator to make a duplicate of the key. The locksmith or proprietor uses his lock and key template guide to determine the type of lock, and he refers to his reference book to find the manufacturer.