I guess it's pointless bringing up the evidence that there exists two or even three wallets purportedly owned by Oswald? It's all because he likes to play James Bond games? How convenient.
As we speak, I can lay hands on three of mine, with a further two at least languishing in various drawers. Why is this significant?
The newest of these is a gift received from my daughters just last Friday. Presumably, in your fantasy world, they are the couriers for my spy network at the tender age of 11 and 14.
Can you not see how ludicrous this whole proposition is?
Say a code phrase and match two parts. From very little light you'll be able to tell that both portions match.
And like any spy, carry the means to incriminate yourself when you are intercepted. Sorry for the fantasy, but there are more sophisticated and subtle methods of validation than that which have been in use since long before JFK.
You can just picture it...
Suspicious Spy: You have your half of the dollar?
Suspicious Handler: What dollar? Are you an agent?
Suspicious Spy: Show me your half dollar and I will show you mine.
Suspicious Handler: Are you trying to entrap me?
And so forth. As a recognition method, it would be mostly useless except in Hollywood.
Think about it for a moment. If spy and handler are known to each other, they don't need it. If spy and handler are unknown to each other, neither will admit to having it. (unless they are daft).
Suppose our covert operative experiences some emergency and gets exposed. He/she then escapes and makes their way to a nearest safe haven, an embassy, whatever. Somehow, the matching half dollar just happens to be present at that particular location as opposed to all of the other possible thousands of locations. Unlikely. Covert ops does not leave such things to chance. It follows that our putative spy must cool his/her heels until the correspond half dollar gets transported to the putative spy's location or vice versa. Using such a method would likely mean that any possible intelligence would be out of date, and there is still no guarantee that the putative agent is not a double carrying what was found on the actual captured and tortured agent and that the identity could easily be verified by other means in a fraction of the time.