50 Cent Coin Knife
They're trying to say that no one will ever suspect an escape knife is nestled within this 50-cent piece, but I think I would suspect that anyone who still carries around a 50-cent piece is up to no good, hidden blade or not. They're like the shady cats who whip out $2 bills or, like, a twenty with Andrew Jackson's head all shrunken to half size to pay for their coffee. Definitely not to be trusted.
But I guess if you want to hide a knife in a coin, JFK's denomination is the best size for it. The covert blade is circular in shape and made of Japanese steel strong enough to cut through rope, zip ties, plastic bondage, duct tape, or any other non-metal material. The coin knife's 2 segments slide apart easily to expose its blade. Access is possible with one hand and with hands bound behind the back. Obviously a downside is that using the knife during a kidnapping or poorly chosen sexual encounter requires that the coin's owner not been robbed and/or had his pants removed before being tied up.