There isn't one. The "key of Solomon" has always been symbolic,
regardless of its context.
The earliest mention of this key are the "clavicles of Solomon"
16-17th century grimmoires (the lemegeton and goetia and I believe
there is a third..) that taught how to summon angels and demons to
one's bidding, supposedly based off the hidden teachings of King
Solomon from the bible.
The most prominent use of the term "key of Solomon" refers to
free masonry. Since the masons teach ethical and moral ideas
through the myth of Hiram Abif (the architect supposedly
responsible for the construction of the temple of King Solomon),
the key of Solomon is the understanding that masonry gives to its
practitioners about the universe and god.