Back in the 1980's, Sam Dalal put out this item, but after a few years, it was no longer available.  Not sure what happened, but I guess he lost the ability to produce it for a reasonable price.   Because it was a very neat gimmick, and puzzling effect.   Well, about 10 years ago, Sam started to produce this trick again.   The one for sale here is one of these newer issues of his borrowed ring on key effect, called KEY TO MY CHEST.

The old-fashion key is shown and handed to the spectator to examine.  The magi comments on her beautiful finger ring, and asks her if she can put her ring on the key.  She tries, but the large round bow at one end is far too large to allow the ring onto the key, and the rectangular teeth near the other end also keep the ring from sliding onto the key shaft.   But nonetheless, the magi takes the key, gently rubs it between his fingers, a la Uri Geller, psychically softening the metal.  He then places the ring on the end of the key, and somehow, it is now on the center of the shaft.   He hands the key back to the spectator so she can verify that it is in fact her ring.   Then with a bit more Geller-esque finger rubbing, the ring somehow comes off the key.

The ring in the photo (not included) is one I grabbed from my wife's jewelry box.  Almost any woman's ring will work.  Some men's rings that are very wide will not work -- my wedding band, for example, cannot be put on the key.  But the effect is best when done for a female spectator anyway.

The gimmicked key is supplied with a ribbon, tied to an old-fashion hotel room number tag.    The effect can be performed without the tag and ribbon, using just the key.

The mechanics of this trick are not complicated, but I will say that it takes some practice to execute this smoothly and to develop a feel for the misdirection. 

Brand new with instructions.