I’ve always assumed that a double-denomination error would necessarily also be a wrong planchet error. After all, I’m not aware of any nation that uses the exact same planchet for two or more of its domestic issues.
Even among dual-country, double-denomination errors, this rule has generally held, since each nation establishes its own coinage specifications, and these are unlikely to match those of any other nation.
Sometimes by intention
I will concede that certain foreign coins are struck on planchets identical to those of another denomination that happens to belong to the nation that strikes both. From 1968 to 1984, the U.S. Mint struck (with periodic interruptions) Panamanian tenth-, quarter- and half-balboa coins on copper-nickel clad planchets that were respectively identical to domestic dimes, quarter dollars, and half dollars.
Had any of these U.S. designs been struck over a Panamanian coin of identical specifications, you would naturally have a double-denomination error without any change in composition, diameter, or weight.
While I’m not aware of any such errors, there are a number of Panamanian 1982 half balboas struck over earlier Kennedy half dollars. In an online article, Ken Glickman provides persuasive evidence that all were intentionally struck (http://kennedyhalfdollar.com/about-halves/blue-balboa-kennedy.html).
Recently, error dealer Fred Weinberg came up with a pair of dual-country, double-denomination errors in which the final strike was on its proper planchet. But the route taken was quite different from the one just described. In each case, the host coin happened to be an off-metal strike on a planchet appropriate for the final strike.
The first coin presented here is a 1983 Canadian cent struck over an Israel 10 shekels that in turn was struck on a Canadian cent planchet. This particular 10-shekel design, which features an ancient galley on the obverse face, was produced in the years 1982 to 1985. Slightly larger than a U.S. quarter dollar, this solid copper-nickel coin was struck at the Royal Canadian Mint.
The second coin is a 1982 Costa Rican 25-centimo piece struck over an Israel 10-shekel coin that was in turn struck on a Costa Rican 25-centimo planchet. Composed of aluminum, the Costa Rican 25-centimo coin of this time period is slightly smaller than a U.S. quarter dollar and was also struck at the Royal Canadian Mint. This is the only dual-country, double-denomination error I’m aware of in which neither strike belongs to the nation actually contracted to do the striking.
Weinberg verified the weights of both planchets before encapsulation.
Likely intentional errors
I strongly suspect that both coins are intentional errors. It’s highly unlikely that a planchet intended for one denomination would be struck by another denomination and then return to the correct production stream in advance of the final strike.
It’s also probably no accident that the Costa Rican 25-centimo strike (and by the looks of it, the Canadian cent strike as well) were struck in-collar. In both coins, the strike of the Israel 10-shekel coin appears to have been of respectable strength. This should have caused the planchets to expand beyond the diameter of a Canadian cent collar and a Costa Rican 25-centimo coin collar, respectively. Consequently, the final strike in each case should have been a broadstrike.
I strongly suspect that, after the Israel strike, each off-metal coin was rolled and squeezed to a smaller diameter that would once again fit into the collar chosen for the final strike. To accomplish this, it’s possible that each off-metal Israel coin was fed into the upset mill appropriate for the planchet it was struck on. By this means, the 10-shekel coin struck on the Canadian cent planchet would have had its diameter reduced to something less than 19.1 millimeters (the diameter of a normal Canadian cent) and the 10-shekel piece struck on the Costa Rican 25-centimo planchet would have had its diameter reduced to slightly less than 22 millimeters (the diameter of a normal 25-centimo piece).
According to the eBay auction descriptions under which these coins appeared, each example was sold to Weinberg by the son of an Israel Mint director. This detail adds even more weight to the argument that these are intentional errors.
When a double-denomination coin is released through normal channels, it is bundled together with other coins that carry the same design as the final strike. So the Canadian error coin should have been discovered in Canada amidst a shipment of Canadian cents and the Costa Rican error coin should have been discovered in Costa Rica amidst a shipment of Costa Rican 25-centimo coins.
The fact that both coins ended up in the hands of an Israel Mint director suggests that they were purposely shipped directly to Israel or even hand-carried across the Atlantic.
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