10 4 zinc is aeny.. see... MD seems to be the std answer, but thinking back to chem101 and what little I know about minting, I think it could be from a weak strike and/or underling deformity combined with a rapid collapse of the copper shell essentially sealing in the "unconformity" to use a geologic term, in essence protecting the coin from rot.
I admit, this is just a wild guess from somebody who doesn't know enough about zinc coins to have an opinion at this point, but it was kinda fun to think about.
I hope you get a real answer amigo as I've wondered this as well.