Magician coins (specifically half dollars)
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or...
Everytying you never wanted to know about the half dollar magicisn'a coin
I was into "magic," as we used to call it, in my younger days.
This half dollar was made in two main varieties and generally marketed as a trick named the "Scotch and Soda." Google it or youtube it.
The English penny was also used instead of a MX peso.
Why the peso and the penny? They are a two of the closest matches overall for the half dollar in size. Thus easy to machine to fit inside the shell. Also it is likley the odd look (to anon-coin collector_ of a large foreign coin is a bit distracting... "I have never sen one of those before..."
Another good trick (done it many times), is to hold the shell, JFK side up, with the other coin showing under it so it looks like you have two coins being held, spread apart,with two fingers. Show it to a person and explain what the other coin is (most don't know). Ask them to extend their hand, quickly place the coins in their hand while pressing them together and closing their fingers over them, simultaneously telling them them to hold it in their closed fist firmaly firmly.
Wave your hand over theirs and ask them to show you both coins. Of course they only find one and are shocked b/c they cannot figure out where the other one went. Even stoic people tend to give a shocked look.
To make it really good, have another peso/penny in your shirt pocket, shoe, whatever to pull out and show them. If you are adept at slight of hand, pull it out of their pocket.
Since coin collectors don't often note the seam on the
Magician's coin, its very unlikely someone who does not look at coins will.
Google "bang Ring" for the device typically used to separate these parts from each other. Or... put it in a sturdy glass and shake it around the bottom until the come apart.
I found ten of these while roll searching 378,000 halves.